Yes, SUBSTANTIVE is a Scrabble word!

Letter values in Scrabble US: S 1, U 1, B 3, S 1, T 1, A 1, N 1, T 1, I 1, V 4, E 1.

Is it a valid word?

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

having a firm basis in reality and being therefore important, meaningful, or considerable
defining rights and duties as opposed to giving the rules by which rights and duties are established
being on topic and prompting thought