Yes, LITRES is a Scrabble word!

Letter values in Scrabble US: L 1, I 1, T 1, R 1, E 1, S 1.

Is it a valid word?

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

a metric unit of capacity, formerly defined as the volume of one kilogram of pure water under standard conditions; now equal to 1,000 cubic centimeters (or approximately 1.75 pints)

Also: l, cubic decimeter, cubic decimetre.