Hello @d1bg12 , thank you for your question!
The error message you are seeing explains what caused your problem. Validation failed because your icon contains transparent pixels. In other words, your icon needs to be totally solid with no see-through pixels.
You can find more information on our documentation page for app icons. For simple icons on iOS, your icon needs to be a non-transparent square image that is 1024x1024 pixels in size. I recommend trying to make the icon yourself in a program like Photoshop or GIMP before relying on a specialized web tool that might not behave the way you expect.
The website you linked to is creating an entire image set for you, which you can use for your icons in Xcode, but this is a bit more involved than just supplying a single texture. You can see our documentation page Adding images to your Xcode project for more information.
For more complex icons, you can also use Icon Composer to design your icon.
Thank you!