Yes, POSTULATOR is a Scrabble word!

Letter values in Scrabble US: P 3, O 1, S 1, T 1, U 1, L 1, A 1, T 1, O 1, R 1.

Is it a valid word?

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

(Roman Catholic Church) someone who proposes or pleads for a candidate for beatification or canonization
someone who assumes or takes something for granted as the basis of an argument