A 1024 x 1024 pixel app icon for your app must be added to the asset catalog in Xcode

same, project has this asset.. What did you do to fix it?

I suddenly experienced this error, but only with the MacOS version. No problem submitting before and no problem submitting iOS version. Just the MacOS version.

Solution is to check "Mac" in the App Icon Asset Catalog and add icons for all sizes.

Don't forget to include alpha channels in the icons for MacOS, but NOT for iOS.

Getting same error. I already added icon of mentioned size.

Solution works for my app (iOS + maccatalyst, and I experienced
 this issue with the maccatalyst version):

1. Open Your project folder -> Images.xcassets -> AppIcon.appiconset -> Contents.json

2. Append below contents into the Contents.json . Details from appendix.

3. Click Project settings -> General -> App Icons Source ,and fill all size icons, including iOS & macOS.

Hope to help.

Appendix
My case:
Origin:

{
 "images" : [
  {
   "filename" : "appIconiOS20@2x.jpg",
   "idiom" : "iphone",
   "scale" : "2x",
   "size" : "20x20"
  },
  {
   "filename" : "appIconiOS20@3x.jpg",
   "idiom" : "iphone",
   "scale" : "3x",
   "size" : "20x20"
  },
  {
   "filename" : "appIconiOS29@2x.jpg",
   "idiom" : "iphone",
   "scale" : "2x",
   "size" : "29x29"
  },
  {
   "filename" : "appIconiOS29@3x.jpg",
   "idiom" : "iphone",
   "scale" : "3x",
   "size" : "29x29"
  },
  {
   "filename" : "appIconiOS40@2x.jpg",
   "idiom" : "iphone",
   "scale" : "2x",
   "size" : "40x40"
  },
  {
   "filename" : "appIconiOS40@3x.jpg",
   "idiom" : "iphone",
   "scale" : "3x",
   "size" : "40x40"
  },
  {
   "filename" : "appIconiOS60@2x.jpg",
   "idiom" : "iphone",
   "scale" : "2x",
   "size" : "60x60"
  },
  {
   "filename" : "appIconiOS60@3x.jpg",
   "idiom" : "iphone",
   "scale" : "3x",
   "size" : "60x60"
  },
  {
   "filename" : "appIconiOS20.jpg",
   "idiom" : "ipad",
   "scale" : "1x",
   "size" : "20x20"
  },
  {
   "filename" : "appIconiOS20@2x-1.jpg",
   "idiom" : "ipad",
   "scale" : "2x",
   "size" : "20x20"
  },
  {
   "filename" : "appIconiOS29.jpg",
   "idiom" : "ipad",
   "scale" : "1x",
   "size" : "29x29"
  },
  {
   "filename" : "appIconiOS29@2x-1.jpg",
   "idiom" : "ipad",
   "scale" : "2x",
   "size" : "29x29"
  },
  {
   "filename" : "appIconiOS40.jpg",
   "idiom" : "ipad",
   "scale" : "1x",
   "size" : "40x40"
  },
  {
   "filename" : "appIconiOS40@2x-1.jpg",
   "idiom" : "ipad",
   "scale" : "2x",
   "size" : "40x40"
  },
  {
   "filename" : "appIconiOS76.jpg",
   "idiom" : "ipad",
   "scale" : "1x",
   "size" : "76x76"
  },
  {
   "filename" : "appIconiOS76@2x.jpg",
   "idiom" : "ipad",
   "scale" : "2x",
   "size" : "76x76"
  },
  {
   "filename" : "appIconiOS83.5@2x.jpg",
   "idiom" : "ipad",
   "scale" : "2x",
   "size" : "83.5x83.5"
  },
  {
   "filename" : "appIconiOS1024.jpg",
   "idiom" : "ios-marketing",
   "scale" : "1x",
   "size" : "1024x1024"
  }
  
 ],
 "info" : {
  "author" : "xcode",
  "version" : 1
 }
}


After appending:

{
 "images" : [
  {
   "filename" : "appIconiOS20@2x.jpg",
   "idiom" : "iphone",
   "scale" : "2x",
   "size" : "20x20"
  },
  {
   "filename" : "appIconiOS20@3x.jpg",
   "idiom" : "iphone",
   "scale" : "3x",
   "size" : "20x20"
  },
  {
   "filename" : "appIconiOS29@2x.jpg",
   "idiom" : "iphone",
   "scale" : "2x",
   "size" : "29x29"
  },
  {
   "filename" : "appIconiOS29@3x.jpg",
   "idiom" : "iphone",
   "scale" : "3x",
   "size" : "29x29"
  },
  {
   "filename" : "appIconiOS40@2x.jpg",
   "idiom" : "iphone",
   "scale" : "2x",
   "size" : "40x40"
  },
  {
   "filename" : "appIconiOS40@3x.jpg",
   "idiom" : "iphone",
   "scale" : "3x",
   "size" : "40x40"
  },
  {
   "filename" : "appIconiOS60@2x.jpg",
   "idiom" : "iphone",
   "scale" : "2x",
   "size" : "60x60"
  },
  {
   "filename" : "appIconiOS60@3x.jpg",
   "idiom" : "iphone",
   "scale" : "3x",
   "size" : "60x60"
  },
  {
   "filename" : "appIconiOS20.jpg",
   "idiom" : "ipad",
   "scale" : "1x",
   "size" : "20x20"
  },
  {
   "filename" : "appIconiOS20@2x-1.jpg",
   "idiom" : "ipad",
   "scale" : "2x",
   "size" : "20x20"
  },
  {
   "filename" : "appIconiOS29.jpg",
   "idiom" : "ipad",
   "scale" : "1x",
   "size" : "29x29"
  },
  {
   "filename" : "appIconiOS29@2x-1.jpg",
   "idiom" : "ipad",
   "scale" : "2x",
   "size" : "29x29"
  },
  {
   "filename" : "appIconiOS40.jpg",
   "idiom" : "ipad",
   "scale" : "1x",
   "size" : "40x40"
  },
  {
   "filename" : "appIconiOS40@2x-1.jpg",
   "idiom" : "ipad",
   "scale" : "2x",
   "size" : "40x40"
  },
  {
   "filename" : "appIconiOS76.jpg",
   "idiom" : "ipad",
   "scale" : "1x",
   "size" : "76x76"
  },
  {
   "filename" : "appIconiOS76@2x.jpg",
   "idiom" : "ipad",
   "scale" : "2x",
   "size" : "76x76"
  },
  {
   "filename" : "appIconiOS83.5@2x.jpg",
   "idiom" : "ipad",
   "scale" : "2x",
   "size" : "83.5x83.5"
  },
  {
   "filename" : "appIconiOS1024.jpg",
   "idiom" : "ios-marketing",
   "scale" : "1x",
   "size" : "1024x1024"
  },
  {
   "filename" : "appIconMacIcon16X16.png",
   "idiom" : "mac",
   "scale" : "1x",
   "size" : "16x16"
  },
  {
   "filename" : "appIconMacIcon16X16@2x.png",
   "idiom" : "mac",
   "scale" : "2x",
   "size" : "16x16"
  },
  {
   "filename" : "appIconMacIcon32X32.png",
   "idiom" : "mac",
   "scale" : "1x",
   "size" : "32x32"
  },
  {
   "filename" : "appIconMacIcon32X32@2x.png",
   "idiom" : "mac",
   "scale" : "2x",
   "size" : "32x32"
  },
  {
   "filename" : "appIconMacIcon128X128.png",
   "idiom" : "mac",
   "scale" : "1x",
   "size" : "128x128"
  },
  {
   "filename" : "appIconMacIcon128X128@2x.png",
   "idiom" : "mac",
   "scale" : "2x",
   "size" : "128x128"
  },
  {
   "filename" : "appIconMacIcon256X256.png",
   "idiom" : "mac",
   "scale" : "1x",
   "size" : "256x256"
  },
  {
   "filename" : "appIconMacIcon256X256@2x.png",
   "idiom" : "mac",
   "scale" : "2x",
   "size" : "256x256"
  },
  {
   "filename" : "appIconMacIcon512X512.png",
   "idiom" : "mac",
   "scale" : "1x",
   "size" : "512x512"
  },
  {
   "filename" : "appIconMacIcon512X512@2x.png",
   "idiom" : "mac",
   "scale" : "2x",
   "size" : "512x512"
  }
 ],
 "info" : {
  "author" : "xcode",
  "version" : 1
 }
}

swebal's solution worked for me. I initially made a new AppIcon asset catalog for Mac, but that didn't work. Instead, you have to select the existing AppIcon catalog, open the Attributes Inspector tab in the right sidebar of Xcode and select the Mac checkbox.

Also, it wasn't necessary to add alpha channels (transparency) to the Mac icons. Maybe it's a problem to have transparency for iOS, but I don't think it's a problem to not have transparency for Mac.

I saw this for macOS Catalyst.
Click attribute inspector, and check Mac. Then you'll be able to add 1024*1024 icon for App Store

So I can confirm what swebal posted 6 days ago works. You get a very generic error for MacOS about 1024x1024 app icon missing even though it is in (Xcode 12) the Assets.xcassets folder as well as in the Preview Content -> Preview Assets.xcassets folder.

I selected all of my images in the Preview Content assets package. Then did a copy. Then I selected the Assets.xcassets folder and did a paste. First time it gave me a bunch a AppIcon.1, Appicon.2 images which I deleted but it DID create the image sizes from the Preview Assets.xcassets folder. I then selected the map icons again as a group, did a copy and then went back into Assets.xcassets folder, selected the new Mac OS sized icons and did a paste. DONE.

Then I also made sure the Attribute for this folder was set for BOTH iOS and MacOS.

I was then able to submit for review with a SUCCESS.

Thanks again to swebal for sharing this.

The suggested solution works, but I notice that icons are no longer rounded. I assume that the previous behavior was inherited from iOS, and that if you want round macCat icons you do it yourself.

same, project has this asset.. What did you do to fix it?

I went back into the assets catalog, and finally I discovered the extra icons needed for Mac after some more scrolling.
Of course Xcode didn't bother to tell me about it - neither upon adding a Mac version to an iOS app nor during "validation" in Xcode or upon Archiving. I wonder if anyone at Apple ever checked went through that process themselves... the easiest things are frustrating problems, extremely unnecessarily so.

If you look carefully, you'll notice that the additional row of Mac icon images doesn't end with a 512 x 512 one. That last one has fine print that specifies a 1024 x 1024 "2x" one.

Unbelievable.

Im getting the error on Xcode 12.2 and I'm not even adding Mac-specific icons, I'm just letting it use the iOS icons. Must be a bug.

In Assets.xcassets --> AppIcon --> show the appicon attribute inspector--> enable the Mac section, then you can see the 1024 icon for mac, add your icon in it and upload the app again

Mine is an iOS app, and it has the 1024x1024 icon, I've checked the resolution, alpha channel, everything.. still getting the error when submitting for review in the App Store (( Everything was fine though with the older Xcode version (11.3.1). What do I do now?

Make sure the 1024x1024 icon is actually those dimensions. A popular icon generator makes the image 1534x1534 and mislabels it 1024x1024