Yes, MANIFESTATION is a Scrabble word!

Letter values in Scrabble US: M 3, A 1, N 1, I 1, F 4, E 1, S 1, T 1, A 1, T 1, I 1, O 1, N 1.

Is it a valid word?

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

a clear appearance
a manifest indication of the existence or presence or nature of some person or thing
an appearance in bodily form (as of a disembodied spirit)
expression without words
a public display of group feelings (usually of a political nature)

Not playable in Scrabble US: MATERIALISATION.