Yes, DELUSION is a Scrabble word!

Letter values in Scrabble US: D 2, E 1, L 1, U 1, S 1, I 1, O 1, N 1.

Is it a valid word?

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

(psychology) an erroneous belief that is held in the face of evidence to the contrary
a mistaken or unfounded opinion or idea
the act of deluding; deception by creating illusory ideas

Also: psychotic belief, head game.