Can't get inheriting TM to work (Big Sur)
I had the same issue.
Seems to me that the -a option of tmutils is broken in Big Sur (11.2.1).
However, instead of letting tmutils perform the association on all volume stores:
sudo tmutil associatedisk -a &quot;/Volumes/Macintosh HD&quot; &quot;/Volumes/Time Machine Backup/Backups.backupdb/Christian Fischer’s iMac/Latest/Macintosh HD&quot;

you can do the association for each volume store yourself:
for f in &quot;/Volumes/Time Machine Backup/Backups.backupdb/Christian Fischer’s iMac/&quot;*; do if [[ -d &quot;$f&quot; ]]; then sudo tmutil associatedisk / &quot;$f/Macintosh HD&quot;; fi; done

This worked for me like a charm ;-)