I've installed Big Sur beta 2 and tried to use Boot Camp Assistant but I got the error "disk could not be partitioned". Also tried to partition manually but "a problem occurred while resizing APFS container structures". I don't use Time Machine but there were 3 snapshots starting with com.apple.os.update- that can't be deleted. I ran First Aid and fsck_apfs and no problem was found. Any other thing to try? Thanks!
Finally found a fix! Deleting snapshots using
tmutil didn't work but I deleted them with
diskutil from Recovery:
diskutil apfs listSnapshots <Macintosh HD's disk id> and then
diskutil apfs deleteSnapshot <disk id> -uuid <snapshot uuid from the previous command> for every snapshot. I had 3, two of them were deleted and one returned "insufficient privileges" but it doesn't matter, now everything works. Do it at your own risk, I don't know whether it will work on your Mac.