Yes, NEGATION is a Scrabble word!

Letter values in Scrabble US: N 1, E 1, G 2, A 1, T 1, I 1, O 1, N 1.

Is it a valid word?

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

a negative statement; a statement that is a refusal or denial of some other statement
the speech act of negating
(logic) a proposition that is true if and only if another proposition is false