I have MacBook Pro 13 inch 256gb 2017 running MacOS Big Sur beta and had the same error until I free my storage up to 40gb and now Xcode 12 is installing from AppStore :)
aminedge
Users receive reputation points by contributing quality content to the forums.
Posts include post and replies published on the forums.
Post authors can mark replies as Accepted to indicate successful solutions.
Apple Developer Forums admins can mark replies as Apple Recommended to indicate an approved solution