Yes, ENDGAME is a Scrabble word!

Letter values in Scrabble US: E 1, N 1, D 2, G 2, A 1, M 3, E 1.

Is it a valid word?

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

the final stages of an extended process of negotiation
the final stages of a chess game after most of the pieces have been removed from the board

Also: end game.