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.