It seems to those who twist the Scriptures to their own destruction, mis-interpret the Bible, and take things out of context, that James is [purportedly] despising prophesying - especially pertaining to predicting the future in the name of the Lord.
Such lamentable heretics legalistically insist (though they themselves often do not do it themselves) that everyone must consistently and in every situation mouth the mantra: "If the Lord wills...". That pathetic prohibition can get a lot of people individually and as a group into serious trouble indeed.
If an employer in a crucial-public-importance business [or an army commander in the military] would hear that phrase, understandable uncertainly would be imposed within the mind of the authority in charge, and he would likely retort: "Well, are you going to be there....or aren't you? Don't give me any of that religious talk! Tell me yes or no NOW without the Bible jargon, or I will fire you now [send you to the brig] on the spot!
Saint Paul wanted all Christians to prophesy (I Corinthians 14:1) for edification (I Corinthians 14:3) so that the church is edified (I Corinthians 14:4) which not only includes discerning the thoughts and intentions of the heart (I Corinthians 14:24-25) but also predicting the future as did the prophets David, Isaiah, Jeremiah, Ezekiel, Daniel, Agabus, and John.
So, First Thessalonians 5:20 orders everyone to not despise prophesying (which - again - includes predicting what will happen tomorrow) in the name of the Lord.
What James is actually concerned with is to not defiantly boast against God's obvious and practical Word, doctrines, and predictions He has already made through his inspired Scripture authors. About those presumptuous pseudo-"prophets" who fabricate their own false and lying portents which never actually happen, James is thinking along the same lines as Ecclesiastes 2:19 which asks "Who knows whether the man to come after will be a wise man or a fool?".....and Ecclesiastes 3:21 which asks "Who knows whether the spirit of man goes upward and the spirit of beasts downward?".....and Ecclesiastes 8:7 stating: "Who knows that which shall be.....?"
Yet we have instances of prophets (coming in from the desert to tent-dweller Abram in Genesis 18:10, and Elijah in II Kings 4:16 provided for by the widow with oil) predicting the birth of a child before the child was conceived, the prophet Jeremiah in Jeremiah 28:15-17 predicting the within-a-year death of heretic prophet Hananiah, Peter in Acts 5:9 predicting the almost immediate death of Sapphira (wife of Ananias, but not the one who laid healing hands on Saul) [although Peter merely stated that the young men would "carry her out" with no specific mention of her death nor how she died, which death might have even shocked him somewhat].
To reiterate, James instructs us to not arrogantly brag against God and God's Biblically-known will - as if God neither could nor would ever stop us from doing whatever against Him. It just so happens that God frequently enough does NOT allow everyone to do every dastardly thing they concoct in their evil imagination then start to do with their wicked mental and physical resources and opportunities.
However, and at least temporarily, God is quite willing already to let us to whatever we will - whether good or evil. His will - then - is both holy and sovereign AND almost indifferently and near-incredibly permissive.
In other words, He wants us to do what is correct and right morally, but He will (at least temporally and only to a certain extent) allow us to do whatever WE will....including what is against His sacred intentions for us as to what we should always want to do, and in fact do.
Finally, Romans 12:2 tells us that "God's will" is the good and acceptable and perfect intentions and actions WE approve, and the stated "will of God" for us regarding abstinence against fornication is found in I Thessalonians 4:3 and regarding giving thanks in all circumstances is found in I Thessalonians 5:18.