It seems my teambuilder is stuck in 6th gen. I.e., Rotom-W can't learn defog. When I give Unaware Clefable Moonblast, it says it can't learn it. I've run into other problems with pokemon not being able to learn moves that they should be able to.
Reminder that PO currently does not support USUM contents, i.e. "Rotom-W can't learn defog" stays true whether you choose Gen 6 or Gen 7. This will be changed when USUM comes out eventually, but PO currently does not support it. Unaware Clefable can learn Moonblast, but the warning you saw mostly likely came from another move that is not compatible. For example, Soft-Boiled can only be learned in Gen 3 without Hidden Abilities and therefore cannot be learned by Unaware Clefable. This also stays true in both Gen 6 and Gen 7, and even in USUM.