Yes, STILLROOM is a Scrabble word!

Letter values in Scrabble US: S 1, T 1, I 1, L 1, L 1, R 1, O 1, O 1, M 3.

Is it a valid word?

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

a pantry or storeroom connected with the kitchen (especially in a large house) for preparing tea and beverages and for storing liquors and preserves and tea etc

Also: still room.