The answer
The Synod of Jerusalem is an Eastern (Byzantine) Orthodox Ecumenical Council which convened in1672 to discuss a variety of different topics. It was during this council that it was decided to add The Third Book of the Maccabees. The Coptic Orthodox Church did not participate in this council and does recognize this book to be amongst the deuterocanonical books. There is no reason for you not to read it as long as you understand that it is not considered a canonical book by our church. The harm is if you are not secure in your understanding of our church teachings and there is a risk of you accepting, adopting, and/or teaching information which is out of the realm of what is considered acceptable.The Prayer of Manasseh can be found in the Septuagint Bible at the end of 2 Chronicles.
Answered by His Eminence Metropolitan Youssef (opens in a new tab)