Thanks TGeorgeMihai it seems the mistake was mine, I zipped the finished folder instead of all of the resulting files which makes the manifest inside a folder and not in root. It's fixed and my zip can be uploaded now
there's one new error, though, it now says this: Web App Manifest's `fullscreen` was of an unexpected type. The webapp.manifest fullscreen is set to true (not a string), so I don't know why this is an unexpected type. Can anyone help me with this?
nevermind that, I changed the fullscreen value to string and it worked! thank you all who helped