How to use tmutil delete on Big Sur, APFS
The Big Sur syntax changed, but you need to specify the volume and the timestamp with -d and -t:
sudo tmutil delete -d /Volumes/Time\ Machine -t 2020-11-18-100936

You don't need to specify the Backups.backupdb folder or the backup machine.  I had done tmutil setdestination /Volumes/Time\ Machine previously (where Time Machine is the name of your drive.)