Yes, USURPATION is a Scrabble word!

Letter values in Scrabble US: U 1, S 1, U 1, R 1, P 3, A 1, T 1, I 1, O 1, N 1.

Is it a valid word?

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

entry to another's property without right or permission
wrongfully seizing and holding (an office or powers) by force (especially the seizure of a throne or supreme authority)