Yes, LOSE is a Scrabble word!

Letter values in Scrabble US: L 1, O 1, S 1, E 1.

Is it a valid word?

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

fail to keep or to maintain; cease to have, either physically or in an abstract sense
fail to win
suffer the loss of a person through death or removal
place (something) where one cannot find it again
miss from one's possessions; lose sight of
allow to go out of sight
fail to make money in a business; make a loss or fail to profit
fail to get or obtain
retreat
fail to perceive or to catch with the senses or the mind
be set at a disadvantage

Also: turn a loss, fall back, drop off, fall behind.