Yes, PROHIBITION is a Scrabble word!

Letter values in Scrabble US: P 3, R 1, O 1, H 4, I 1, B 3, I 1, T 1, I 1, O 1, N 1.

Is it a valid word?

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

a law forbidding the sale of alcoholic beverages
a decree that prohibits something
the period from 1920 to 1933 when the sale of alcoholic beverages was prohibited in the United States by a constitutional amendment
refusal to approve or assent to
the action of prohibiting or inhibiting or forbidding (or an instance thereof)

Also: prohibition era.