Yes, STEWARD is a Scrabble word!

Letter values in Scrabble US: S 1, T 1, E 1, W 4, A 1, R 1, D 2.

Is it a valid word?

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

someone who manages property or other affairs for someone else
the ship's officer who is in charge of provisions and dining arrangements
an attendant on an airplane
a union member who is elected to represent fellow workers in negotiating with management
one having charge of buildings or grounds or animals

Also: flight attendant, shop steward.