Eric converted to High Sierra on his Mac and chose APFS on Time Machine. Now he's got trouble. Leo says that Apple cautioned not to do that. Can he at least downgrade? Leo says that the good news is there was no data lost, so he could format it to downgrade from APFS and then let it backup again. Eric should check out this article at macobserver on how to use Time Machine with APFS. He shouldn't convert to APFS if he has a Fusion drive just yet.
Leo's not a fan of Time Machine in general. He thinks it's a kludge of technology that is really not an elegant backup option. Is it any wonder APFS doesn't like it? Leo recommends the program SuperDuper. It'll not only work with Time Machine, but it can create a bootable version of the backup should he have problems.