Yes, SUMMATION is a Scrabble word!

Letter values in Scrabble US: S 1, U 1, M 3, M 3, A 1, T 1, I 1, O 1, N 1.

Is it a valid word?

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

a concluding summary (as in presenting a case before a law court)
(physiology) the process whereby multiple stimuli can produce a response (in a muscle or nerve or other part) that one stimulus alone does not produce
the final aggregate
the arithmetic operation of summing; calculating the sum of two or more numbers

Also: summing up, sum total.