Time Machine fails since enabling FileVault on iMac with Fusion Drive
Solution 1:
The problem isn't the TM disk encryption, but the system disk Data "Macintosh HD - Data". You have to wait that the filevault encryption be full.
If your Mac is a laptop model, keep in mind that the encryption requires an electrical connection.
To verify the encryption 2 commands possible :
diskutil ap list or
fdesetup status