Yes, CUBITUS is a Scrabble word!

Letter values in Scrabble US: C 3, U 1, B 3, I 1, T 1, U 1, S 1.

Is it a valid word?

CUBITUS is a valid word in the official Scrabble dictionary (US) and the UK Scrabble Dictionary, but not in Words with Friends (ENABLE).

hinge joint between the forearm and upper arm and the corresponding joint in the forelimb of a quadruped
the arm from the elbow to the fingertips

Also: elbow joint, human elbow, cubital joint, articulatio cubiti.