Can I use Clefable's "Moonlit Miracle" attack if the only Pokemon I have in play cannot evolve?
You can use the attack no matter what, but after flipping coins if you have no Pokemon that can evolve the attack stops there. You wouldn't get to search your deck since you know there won't be any effect.
Source: Brilliant Stars FAQ; TPCi Rules Team (2022-02-24)

Relevant cards

Note: All applicable cards may not be listed below.
SWSH9_55
SWSH9_55