***The Epistle of James should not be listed with the canonical books of the New Testament.***
The answer is simple.
Paul was writing to the Gentile believers as he was the Apostlt to the Gentiles.
James was writing “To the twelve tribes scattered abroad”. As on can see at the Jerusalem meeting between James and Paul, James still wanted the Jewish believers to keep the LAW of MOSES while Paul’s Gentile believers were free from that LAW.
You can’t mix Law and Grace. You can’t mix James and Paul.
When 2 Tm 3:16 says, All Scripture is given by inspiration of God, and is profitable for doctrine, for reproof, for correction, for instruction in righteousness, that the man of God may be complete, thoroughly equipped for every good work., does that include the Epistle of James?