Yes, MANGOSTEENS is a Scrabble word!

Letter values in Scrabble US: M 3, A 1, N 1, G 2, O 1, S 1, T 1, E 1, E 1, N 1, S 1.

Is it a valid word?

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

East Indian tree with thick leathery leaves and edible fruit
two- to three-inch tropical fruit with juicy flesh suggestive of both peaches and pineapples

Also: mangosteen tree, garcinia mangostana.