Yes, SINKHOLE is a Scrabble word!

Letter values in Scrabble US: S 1, I 1, N 1, K 5, H 4, O 1, L 1, E 1.

Is it a valid word?

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

a depression in the ground communicating with a subterranean passage (especially in limestone) and formed by solution or by collapse of a cavern roof

Also: swallow hole.