Yes, RABBIS is a Scrabble word!

Letter values in Scrabble US: R 1, A 1, B 3, B 3, I 1, S 1.

Is it a valid word?

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

spiritual leader of a Jewish congregation; qualified to expound and apply Jewish law
a Hebrew title of respect for a Jewish scholar or teacher