Yes, PRESUMPTION is a Scrabble word!

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

Is it a valid word?

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

an assumption that is taken for granted
(law) an inference of the truth of a fact from other facts proved or admitted or judicially noticed
audacious (even arrogant) behavior that you have no right to
a kind of discourtesy in the form of an act of presuming

Also: presumptuousness.