Yes, CRITICAL is a Scrabble word!

Letter values in Scrabble US: C 3, R 1, I 1, T 1, I 1, C 3, A 1, L 1.

Is it a valid word?

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

marked by a tendency to find and call attention to errors and flaws
at or of a point at which a property or phenomenon suffers an abrupt change especially having enough mass to sustain a chain reaction
characterized by careful evaluation and judgment
urgently needed; absolutely necessary
forming or having the nature of a turning point or crisis
being in or verging on a state of crisis or emergency
of or involving or characteristic of critics or criticism