2 Kings 24:4 (ESV)

4 and also for the innocent blood that he had shed. For he filled Jerusalem with innocent blood, and the LORD would not pardon.

Concerning the unwillingness to forgive, see Deuteronomy 29:19 where it also states the Lord’s intention not to forgive. However, that is not absolute; see Deuteronomy 30:1–10 where there can be forgiveness after repentance. Did Manasseh not repent? Isn’t that what we read about in 2 Chronicles 33:12–19? It is questionable whether this really was a conversion of the heart, because we read especially that he then took into account the Lord’s actions, because he had personally experienced his power. The Bible’s judgment of Manasseh remains very negative.