Yes, RULE is a Scrabble word!

Letter values in Scrabble US: R 1, U 1, L 1, E 1.

Is it a valid word?

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

a principle or condition that customarily governs behavior
something regarded as a normative example
prescribed guide for conduct or action
(linguistics) a rule describing (or prescribing) a linguistic practice
a basic generalization that is accepted as true and that can be used as a basis for reasoning or conduct
the duration of a monarch's or government's power
dominance or power through legal authority
directions that define the way a game or sport is to be conducted
any one of a systematic body of regulations defining the way of life of members of a religious order
a rule or law concerning a natural phenomenon or the function of a complex system
(mathematics) a standard procedure for solving a class of mathematical problems
measuring stick consisting of a strip of wood or metal or plastic with a straight edge that is used for drawing straight lines and measuring lengths

Also: linguistic rule.