Yes, SYNCRETIC is a Scrabble word!

Letter values in Scrabble US: S 1, Y 4, N 1, C 3, R 1, E 1, T 1, I 1, C 3.

Is it a valid word?

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

relating to a historical tendency for a language to reduce its use of inflections
of or characterized by syncretism

Not playable in Scrabble US: SYNCRETICAL, SYNCRETISTICAL.