Yes, CANASTA is a Scrabble word!

Letter values in Scrabble US: C 3, A 1, N 1, A 1, S 1, T 1, A 1.

Is it a valid word?

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

a form of rummy using two decks of cards and four jokers; jokers and deuces are wild; the object is to form groups of the same rank

Also: basket rummy.