Yes, PREPOSITION is a Scrabble word!

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

Is it a valid word?

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

a function word that combines with a noun or pronoun or noun phrase to form a prepositional phrase that can have an adverbial or adjectival relation to some other word
(linguistics) the placing of one linguistic element before another (as placing a modifier before the word it modifies in a sentence or placing an affix before the base to which it is attached)