Yes, PRECURSORS is a Scrabble word!

Letter values in Scrabble US: P 3, R 1, E 1, C 3, U 1, R 1, S 1, O 1, R 1, S 1.

Is it a valid word?

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

a substance from which another substance is formed (especially by a metabolic reaction)
a person who goes before or announces the coming of another
something that precedes and indicates the approach of something or someone