Yes, DECLINE is a Scrabble word!

Letter values in Scrabble US: D 2, E 1, C 3, L 1, I 1, N 1, E 1.

Is it a valid word?

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

change toward something smaller or lower
a condition inferior to an earlier condition; a gradual falling off from a better state
a gradual decrease; as of stored charge or current
a downward slope or bend

grow worse
refuse to accept
show unwillingness towards
grow smaller
go down
go down in value
inflect for number, gender, case, etc., "in many languages, speakers decline nouns, pronouns, and adjectives"

Also: pass up, turn down, go down.