Yes, COMMONWEALTHS is a Scrabble word!

Letter values in Scrabble US: C 3, O 1, M 3, M 3, O 1, N 1, W 4, E 1, A 1, L 1, T 1, H 4, S 1.

Is it a valid word?

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

the official name of some states in the United States (Massachusetts and Pennsylvania and Virginia and Kentucky) and associated territories (Puerto Rico)
a politically organized body of people under a single government
a world organization of autonomous states that are united in allegiance to a central power but are not subordinate to it or to one another
a political system in which the supreme power lies in a body of citizens who can elect people to represent them

Also: res publica, body politic.