Yes, DEMERIT is a Scrabble word!

Letter values in Scrabble US: D 2, E 1, M 3, E 1, R 1, I 1, T 1.

Is it a valid word?

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

a mark against a person for misconduct or failure; usually given in school or armed forces
the quality of being inadequate or falling short of perfection