He found that you can’t install the beta on the same APFS partition you used for previous versions of macOS. If you do, you’ll see a warning like this:
You’ll also see an error about an incompatible disk:
People on the linked Reddit thread mentioned it could depend on whether you install the beta on a separate volume on a partition.
Currently we’re on the first developer beta, so bugs like this are to be expected (well, maybe not bugs specifically like this).