Yes, SCALLYWAG is a Scrabble word!

Letter values in Scrabble US: S 1, C 3, A 1, L 1, L 1, Y 4, W 4, A 1, G 2.

Is it a valid word?

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

a white Southerner who supported Reconstruction policies after the American Civil War (usually for self-interest)
a deceitful and unreliable scoundrel
one who is playfully mischievous