Yes, MERCANTILISMS is a Scrabble word!

Letter values in Scrabble US: M 3, E 1, R 1, C 3, A 1, N 1, T 1, I 1, L 1, I 1, S 1, M 3, S 1.

Is it a valid word?

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

an economic system (Europe in 18th century) to increase a nation's wealth by government regulation of all of the nation's commercial interests
transactions (sales and purchases) having the objective of supplying commodities (goods and services)

Also: mercantile system.