The answer by tubedogg to question Reuse Time Machine backup drive after migration to new machine described a procedure to do this. It didn't address this issue:
JT-imac:/Users/jt>> sudo tmutil associatedisk -a "/Volumes/Macintosh HD" "Volumes/IoTMBU/Backups.backupdb/Latest/Macintosh HD"
/Users/jt/Volumes/IoTMBU/Backups.backupdb/Latest/Macintosh HD: Not a snapshot volume.
Stupid typo or fouled up TM BU drive?
Best Answer
You're missing a / from the start of your second path, so it's looking in the current directory for Volumes, as shown by the error "/Users/jt/Volumes/…". Add a / to the start of the second path.