Yes, LAMENTATIONS is a Scrabble word!

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

Is it a valid word?

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

an Old Testament book lamenting the desolation of Judah after the destruction of Jerusalem in 586 BC; traditionally attributed to the prophet Jeremiah
a cry of sorrow and grief
the passionate and demonstrative activity of expressing grief

Also: book of lamentations.