Yes, HARMONY is a Scrabble word!

Letter values in Scrabble US: H 4, A 1, R 1, M 3, O 1, N 1, Y 4.

Is it a valid word?

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

compatibility in opinion and action
the structure of music with respect to the composition and progression of chords
a harmonious state of things in general and of their properties (as of colors and sounds); congruity of parts with one another and with the whole
agreement of opinions
an agreeable sound property

Also: musical harmony.