Thanks for your question.
The Prayer of Manasseh is a short work of 15 verses of the penitential prayer of king Manasseh of Judah. Manasseh is recorded in the Bible as one of the most idolatrous kings of Judah (2 Kings 21:1-18; 2 Chronicles 33:1-9). Chronicles, but not Kings, records that Manasseh was taken captive by the Assyrians. (2 Chronicles 33:11-13) While a prisoner, Manasseh prayed for mercy, and upon being freed and restored to the throne turned from his idolatrous ways. (2 Chronicles 33:15-17) A reference to the prayer, but not the prayer itself, is made in 2 Chronicles 33:19, which says that the prayer is written in the "history of the seers." [from Wikipedia]
Since we do not have the "history of the seers" the actual "Prayer of Manasseh" is "apocryphal" and is therefore not included in the protestant Bible.
I hope you have found this helpful.