Are you sure the message is just not telling you that it has run out of space and thus will be deleting older information? I have gotten the message I just described, and it scares you/me.
If indeed for some reason you can't get TM to work on this drive: You could re-format the drive and start all over with a new, first time, backup (which will take quite some time). Another suggestion is to get a new drive and make a new backup. Eventually the first drive will die, at least this way you start its replacement before the old one dies.
I found this link. It says that even though TM deletes old backups, you might run out of space on the drive.
https://support.apple.com/guide/mac-help/if-the-time-machine-backup-disk-is-full-mh15137/mac
I too thought I would never run out, but be asked that I might have to delete some information from older backups.