Yes, ASSUMPTION is a Scrabble word!

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

Is it a valid word?

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

a statement that is assumed to be true and from which a conclusion can be drawn
a hypothesis that is taken for granted
the act of taking possession of or power over something
celebration in the Roman Catholic Church of the Virgin Mary's being taken up into heaven when her earthly life ended; corresponds to the Dormition in the Eastern Orthodox Church
(Christianity) the taking up of the body and soul of the Virgin Mary when her earthly life had ended
audacious (even arrogant) behavior that you have no right to
the act of assuming or taking for granted

Also: laying claim, assumption of mary, august 15, presumptuousness.