Yes, FEUDALISMS is a Scrabble word!

Letter values in Scrabble US: F 4, E 1, U 1, D 2, A 1, L 1, I 1, S 1, M 3, S 1.

Is it a valid word?

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

the social system that developed in Europe in the 8th century; vassals were protected by lords who they had to serve in war

Also: feudal system.