The apple design documentation says that for theme based app icons we should provide the:
1. "You provide a transparent dark icon."
2. "The system provides the gradient background."
3. "The system composites your dark icon on the gradient background."

So how are we supposed to provide a transparent icon if the build systems is throwing the error
"The large app icon in the asset catalog in “Runner.app” can’t be transparent or contain an alpha channel".

This just doesn't make any sense to me.

Providing a white or dark background doesn't apply to the described method to make the iOS handle this use case for us. Odd!

Reference: https://developer.apple.com/design/human-interface-guidelines/app-icons#Platform-considerations