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?
"Volumes/IoTMBU…
should be"/Volumes/IoTMBU…
. – duskwuff -inactive- Sep 1 '17 at 17:53