Instead of uploading the app through Xcode's Organizer, I just exported in from there and then used the Transporter App to upload to the App Store. It gives a much more detailed error message. In my case, it wasn't satisfied with the Build Number that I gave and found a duplicate. I was able to resolve that by uploading through the Transporter App. Transporter App can be downloaded here: https://apps.apple.com/de/app/transporter/id1450874784?l=en-GB&mt=12

Also happened to me once with code that I downloaded from GitHub instead of cloning it. When downloading, many Browsers add the com.apple.quarantine attribure to the files, which will be rejected by the App Store. You can remove it by running this command: xattr -dr com.apple.quarantine /path/to/the/folder.