Yes, SUBSTITUTION is a Scrabble word!

Letter values in Scrabble US: S 1, U 1, B 3, S 1, T 1, I 1, T 1, U 1, T 1, I 1, O 1, N 1.

Is it a valid word?

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

an event in which one thing is substituted for another
the act of putting one thing or person in the place of another: "he sent Smith in for Jones but the substitution came too late to help"