Yes, DEVALUATION is a Scrabble word!

Letter values in Scrabble US: D 2, E 1, V 4, A 1, L 1, U 1, A 1, T 1, I 1, O 1, N 1.

Is it a valid word?

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

an official lowering of a nation's currency; a decrease in the value of a country's currency relative to that of foreign countries
the reduction of something's value or worth