Yes, WEAKNESS is a Scrabble word!

Letter values in Scrabble US: W 4, E 1, A 1, K 5, N 1, E 1, S 1, S 1.

Is it a valid word?

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

a flaw or weak point
powerlessness revealed by an inability to act
the property of lacking physical or mental strength; liability to failure under pressure or stress or strain
the condition of being financially weak
a penchant for something even though it might not be good for you