Beta builds are available only via AutoUpdate.
Things you might try:
Verify your Mac's startup volume:
How to repair a Mac storage device with Disk Utility
Reinstall AutoUpdate:
Download a copy of the current version of AutoUpdate from this link, install and then run AutoUpdate to bring your installation to current update level, then restart your Mac. If the updater does not open automatically after download, double-click the downloaded .pkg file. Click this link to download the updater. The update is free.
https://go.microsoft.com/fwlink/p/?linkid=830196
Boot your Mac to Safe Mode and then try to update again
How to Boot in Safe Mode on an Apple Silicon Powered Mac
Older macs: Perform a Safe Boot by holding the SHIFT key down as your Mac starts up from a restart or shut down.