codesign not signing .app MacOS executable 'can't verify signature'

If you’re signing stuff by hand, see Creating Distribution-Signed Code for Mac for general instructions. That covers most of what’s discussed here but I’ll address some specific points below.

Do we have to sign each individual file and folder?

No. Rather, you should identify each code item and sign that from the inside out. And if the code item is wrapped in a bundle, just sign the bundle. That will seal over the bundle’s main executable, its resources, and any nested code (which you will have already signed because of the inside out rule).

Content/MacOS contains multiple files, including the app executable. These files are signed except the main executable.

Are these other files executables? If not, they’re in the wrong place. See Placing Content in a Bundle for specific guidance on that.

Putting non-executables in Contents/MacOS is a very common source of weird code signing problems.

Another thing to look out for here is that your bundle correctly identifies its main executable via the CFBundleExecutable property in the Info.plist. If you have something weird there, like a misspelling or a value with the wrong case, you’ll see some very odd behaviour.

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Quinn “The Eskimo!” @ Developer Technical Support @ Apple
let myEmail = "eskimo" + "1" + "@" + "apple.com"

Thanks @eskimo

Tried all the steps above. Can you please share any internet article or video to perform given steps.

Regards