Yes, FLATFISH is a Scrabble word!

Letter values in Scrabble US: F 4, L 1, A 1, T 1, F 4, I 1, S 1, H 4.

Is it a valid word?

FLATFISH is a valid word in the official Scrabble dictionary (US), the UK Scrabble Dictionary and Words with Friends (ENABLE).

sweet lean whitish flesh of any of numerous thin-bodied fish; usually served as thin fillets
any of several families of fishes having flattened bodies that swim along the sea floor on one side of the body with both eyes on the upper side