Yes, PREVALENCES is a Scrabble word!

Letter values in Scrabble US: P 3, R 1, E 1, V 4, A 1, L 1, E 1, N 1, C 3, E 1, S 1.

Is it a valid word?

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

the quality of prevailing generally; being widespread
(epidemiology) the ratio (for a given time period) of the number of occurrences of a disease or event to the number of units at risk in the population
a superiority in numbers or amount