Yes, MISSION is a Scrabble word!

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

Is it a valid word?

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

an organization of missionaries in a foreign land sent to carry on religious work
an operation that is assigned by a higher headquarters
a special assignment that is given to a person or group
the organized work of a religious missionary
a group of representatives or delegates

Also: missionary post, missionary station, foreign mission, military mission, missionary work.