It's a common question, with lots of good threads on it. Here's one.

The TLDR is that the North didn't really want outside help (the king of Siam offered war elephants!). and while the South was desperate for foreign aid, no foreign power was willing to risk the costs of angering the much more populous and industrialized North.

You may read from time to time that the Vatican (or the papacy, depending on the source) recognized the Confederacy. Pius IX did write a letter to Jefferson Davis. In that letter, he did address Davis as the “[I]llustrious and honorable President of the Confederate States of America.” However, the full reality is not nearly so dramatic. This is the only place online that I have found the letter from Pius IX to Davis. It is a PDF, and it also comes from a less-than-stellar source. Ignore the rest of the content, just scroll down to the text if you want to read the letter. In sum, the letter is the equivalent of a form letter. It acknowledges receipt of Davis’ earlier letter, says that it would be really nice if there was peace instead of war, asserts that the pope will pray for peace, and that he will also pray that Davis will be granted grace.

That’s it.

So, why the address as “President?” Basically, the pontiff filled in the blank by looking at the signature of the original letter from Davis. With Italian unification and being a self-proclaimed “prisoner of the Vatican,” Pius IX had much greater concerns much closer to home than the Confederacy. Also, the idea that the pope would somehow back the South in the US Civil War is far-fetched, given the less than ideal way Catholics were treated in that region. So, if you hear someone claim that the pope recognized the Confederacy, refer them to this post so they can learn that the truth was much more banal—the pope just replied to Davis’ letter with pleasantries. The only things this letter accomplished was to annoy the North (especially Congress) when it was leaked to the press, and to give revisionist historians a weak ability to appeal to the authority of the pope so long as you don't look closely at the source.