Yes, UNTRUEST is a Scrabble word!

Letter values in Scrabble US: U 1, N 1, T 1, R 1, U 1, E 1, S 1, T 1.

Is it a valid word?

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

not according with the facts
not true to an obligation or trust
not accurately fitted; not level
(used especially of persons) not dependable in devotion or affection; unfaithful

Also: out of true.