I had the same issue when trying to update to Beta 4 on my MBP with M1 chip.
I managed to update using the above-mentioned command line, but I find it strange it's necessary to circumvent the usual update process, can it be related to the M1 chip?
The Beta updates on the ATV, iPhone and iPad went well without any issues although it's not related to Monterey.
Prorege
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You can try this command line (worked for me): https://developer.apple.com/forums/thread/683913