Does prayer become nullified if the transitional takbir is not completed, such as saying "Allahu ak-" when prostrating without finishing it?

The prayer is not invalidated by intentionally or unintentionally omitting the transitional takbeers, according to the majority of scholars, who consider them to be recommended. Your omission of some of one of the takbeers because you have already prostrated does not affect your prayer or invalidate it.