Yes, DUCK is a Scrabble word!

Letter values in Scrabble US: D 2, U 1, C 3, K 5.

Is it a valid word?

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

small wild or domesticated web-footed broad-billed swimming bird usually having a depressed body and short legs
(cricket) a score of nothing by a batsman
flesh of a duck (domestic or wild)
a heavy cotton fabric of plain weave; used for clothing and tents

to move (the head or body) quickly downwards or away
submerge or plunge suddenly
dip into a liquid
avoid or try to avoid fulfilling, answering, or performing (duties, questions, or issues)

Also: duck's egg, put off.