Troubleshooting guide
How to fix the problem
Connect the backup disk and select Add Backup Disk in System Settings > General > Time Machine. If you are prompted to erase the drive, save its existing files first. You can enable copy encryption and save your password securely. Wait for the end of the first copy and check the recovery of at least one file.
If the problem persists
Make sure the first copy is complete and the test file can be restored from it. If the wizard asks you to erase the disk, check that it does not contain a single copy of other files.
Request a repair