Problems selecting backup drive on Time Machine ("Still reverting")

It appears that you elected to disable FileVault encryption for that drive; is that correct? If that's the case, you must wait for the drive to finish de-encrypting before you can use it again if you want to retain its existing data.

It will take about as much time to de-encrypt as it does to encrypt... assuming you encrypted it subsequent to initially formatting it. That can take days—a function of the drive's capacity.

If you are unwilling to wait that long for Time Machine to resume, your options are to obtain and use an additional, redundant backup disk and add it to Time Machine, or to forego the information on that disk and erase it. One and only one backup isn't sufficient anyway.

For future reference: after restoring a system from its TM backup, it's not necessary to de-encrypt or to do anything else for that matter. TM will pick up where it left off, although that initial backup will take a long time while TM figures out what needs to be backed up and what doesn't.

It will... eventually... assuming the disk is intact. And if it isn't, it shouldn't be relied upon anyway.

All disks fail... eventually. That's another reason one and only one backup isn't sufficient.