I can't seem to find information on this Time Machine error message...

I suspect the disk was not properly formatted for Time Machine.

Start with Connect a new backup disk to your Mac. If you did not get the dialog asking if you would like to use that disk for Time Machine when you connected it for the first time, reformat it: Types of disks you can use with Time Machine on Mac.

"APFS or APFS Encrypted disks are the preferred format for a Time Machine backup disk. If you select a new backup disk that’s not already formatted as an APFS disk, you get the option to erase and reformat it. If the disk is a Mac OS Extended format disk that contains an existing Time Machine backup, you aren’t asked to erase and reformat the disk."

To erase and reformat a disk please read Erase and reformat a storage device in Disk Utility on Mac.

Do it anyway.

Encrypting a backup disk takes a long time but it does not affect the ability to use Time Machine while it is encrypting. Any backups created after encryption starts will be encrypted.

If it is an 8 TB disk as its name implies, it will probably take a week or two to encrypt.