Yes, NUMBNESS is a Scrabble word!

Letter values in Scrabble US: N 1, U 1, M 3, B 3, N 1, E 1, S 1, S 1.

Is it a valid word?

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

partial or total lack of sensation in a part of the body; a symptom of nerve damage or dysfunction
the trait of lacking enthusiasm for or interest in things generally