TimeMachine excluded disk problem

iMac, Tahoe 26.5.2

I've run out of internal SSD space on my iMac (Intel / 2020). I attached an external SSD. TimeMachine records the new SSD as "excluded" from backup. I looked into an old (2022) thread, "Time Machine backups exclude problem," and followed the directions. My external SSD is on the excluded list. I followed the directions in the old thread, clicked on my external SSD .... BUT, the '-' / remove from list does not highlight. So, I cannot remove the external SSD from the excluded list.

So, what do I do now?