Yes, PERFECTIVE is a Scrabble word!

Letter values in Scrabble US: P 3, E 1, R 1, F 4, E 1, C 3, T 1, I 1, V 4, E 1.

Is it a valid word?

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

a tense of verbs used in describing action that has been completed (sometimes regarded as perfective aspect)
the aspect of a verb that expresses a completed action

Also: perfective tense, perfect tense, perfective aspect.