I click "upload file", select my apk file, and an error appears: There was a problem completing your upload. Why?  This file is normal and it weighs less than 1 GB.  Or what file is needed to upload a game for android?

Try reuploading a few times, I've had instances where it doesn't work initially when it comes to APK files but randomly it does every other try, so give it a few tries, :o

It's possible that your internet connection is too slow, and the upload times-out before it is finished. You can solve that by uploading it to Dropbox first, and then transferring it from Dropbox to itch.io.