The WorkspaceSettings.xcsettings file is generated with the following entry
<key>BuildSystemType</key> <string>Original</string>
This causes the warning in [Link Removed] which has was closed by suppressing the warning, but Xcode 13 beta 4 no longer seems to allow the legacy build system.
rdar://problem/78763419
A Legacy Build System error will occur.
Note to change back to the regular Xcode in Applications, run the command sudo xcode-select -s /Applications/Xcode.app/Contents/Developer
There's no existing public thread on this issue, so head over to Questions & Answers just mention UE-120841 in the post.