I want to upload my JVM macOS App to Testflight and get this error:

Error: Asset validation failed Missing required icon. The application bundle does not contain an icon in ICNS format, containing both a 512x512 and a 512x512@2x image.

How do I need to add this icon?

Try this: https://github.com/JetBrains/compose-multiplatform/blob/master/tutorials/Native_distributions_and_local_execution/README.md#platform-specific-options

I do that, but Testflight complains. 🤷‍♂️

Did you include the two icons as the error message asked?

It allows two icons?

Inside the icns there are a bunch of icons

Okay, thank you. I will try that. I got the ICNS from my designer and there is indeed only one size included.