Yes, STERILITY is a Scrabble word!

Letter values in Scrabble US: S 1, T 1, E 1, R 1, I 1, L 1, I 1, T 1, Y 4.

Is it a valid word?

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

(of non-living objects) the state of being free of pathogenic organisms
the state of being unable to produce offspring; in a woman it is an inability to conceive; in a man it is an inability to impregnate

Not playable in Scrabble US: STERILENESS.