Yes, INCOMPATIBILITY is a Scrabble word!

Letter values in Scrabble US: I 1, N 1, C 3, O 1, M 3, P 3, A 1, T 1, I 1, B 3, I 1, L 1, I 1, T 1, Y 4.

Is it a valid word?

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

the relation between propositions that cannot both be true at the same time
(immunology) the degree to which the body's immune system will try to reject foreign material (as transfused blood or transplanted tissue)
the quality of being unable to exist or work in congenial combination

Also: mutual exclusiveness.