Yes, POSTPOSITION is a Scrabble word!

Letter values in Scrabble US: P 3, O 1, S 1, T 1, P 3, O 1, S 1, I 1, T 1, I 1, O 1, N 1.

Is it a valid word?

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

(linguistics) the placing of one linguistic element after another (as placing a modifier after the word that it modifies in a sentence or placing an affix after the base to which it is attached)