It looks like it's just Apple Servers being down / overloaded / having issues. It randomly worked for me once earlier today after giving this error a few times, then now I'm still getting the error repeatedly, and the one time it got past this error it just failed uploading later with another issue. Just try again in a few hours would be my best guess....

I have the same problem - note, that to download the old Xcode 12.2, you can't click the link that was shared, you have to go to like https://developer.apple.com/download/more/?=xcode and download it there. One 'solution' is to switch to using the static version of the library, if that's available. The problem in my case is that this cripples the development speed - using the equivalent static lib adds 20 seconds to the link time, taking a 4 second incremental build up to like 24 seconds.