I asked the C3 forums and I was suggested to ask here, does someone know why it does this? And how do I fix it? Thanks a lot!

What you upload to Itch is the same thing that the user downloads, and Itch doesn't manage the installation of APKs, so there's not much you can do on Itch's side.

The only problem I can think of is that if you use buttler, maybe the file is compressed as a . zip, but when downloading on android, you should see the . zip and not an apk.

Try transferring the APK you create in construct directly from your PC to Android and install it, if the error persists, then it's a problem with the APK creation, and it will be much easier for them to help you on the C3 forum.

Check that what you download from Itch is the APK and not a ZIP.

It could be a setting. For example, if you selected Android version 11+ inside Construct and your phone is Android 10 it won't install. Android phones tend to give idiotic, rather than helpful messages when it can't install an app.

By the way, the Google verification thing is simply a form you fill in to state who you are and where you live. It wasn't complicated. That in itself shouldn't have gotten your account closed unless you simply ignored the request.