Yes, NONEXTANT is a Scrabble word!

Letter values in Scrabble US: N 1, O 1, N 1, E 1, X 8, T 1, A 1, N 1, T 1.

Is it a valid word?

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

no longer in existence; lost or especially having died out leaving no living representatives
no longer existing or inaccessible through loss or destruction