Yes, AMORTIZATION is a Scrabble word!

Letter values in Scrabble US: A 1, M 3, O 1, R 1, T 1, I 1, Z 10, A 1, T 1, I 1, O 1, N 1.

Is it a valid word?

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

the reduction of the value of an asset by prorating its cost over a period of years
payment of an obligation in a series of installments or transfers

Not playable in Scrabble US: AMORTISATION.