Is the Prayer of Manesseh canonical or not?
Prayer of Manasseh: This penitential prayer is the prayer mentioned in the Holy Book of 2 Chronicles 33:18. It was not canonized at some councils because it contains the line "For Abraham, Isaac, and Jacob, who did not sin against you..." which contradicts the Biblical accounts of the patriarchs and the doctrine of original sin. It was, however, included in the post-Tridentine appendix to the official Vulgate. The prayer was incorporated in the ancient so-called Apostolic Constitutions.
Of course this line "For Abraham, Isaac, and Jacob, who did not sin against you..." does not mean they did not sin at all, but it means that the final outcome of their lives was according to God's law and thus they are considered righteous.