I get the same error. For some reason, Apple did not update the Command Line Tools with the new beta release. In general, I believe that for every new macOS Big Sur beta release, there is a new Xcode release which comes with updated CLT. Every single time when I updated macOS, I had to grab the fresh Command Line Tools, but for some reason, it is not even listed under the Beta "More" page on developer.apple.com. Basically, it wasn't uploaded. Right now the best bet is probably to download the full Xcode 12 beta 6 release, which supposedly comes with the CLT beta 6 release which is missing. Update: As it seems, the previous CLT works well with the new beta release, though after installing it it simply deletes it. xcode-select also looks for the new release, hence the error, but if one manually installs the CLT beta 5 again, it works just fine. It can be found at the developer downloads - https://developer.apple.com/download/more/