Yes, AMBIGUOUS is a Scrabble word!

Letter values in Scrabble US: A 1, M 3, B 3, I 1, G 2, U 1, O 1, U 1, S 1.

Is it a valid word?

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

open to two or more interpretations; or of uncertain nature or significance; or (often) intended to mislead
having more than one possible meaning
having no intrinsic or objective meaning; not organized in conventional patterns