Nowhere in the documentation does it state that the column headers of your CSV have to have NO special characters, but this is true. Not even a simple hyphen is allowed.
However, instead of informing the user of this fact, the system just sits and spins on the "importing" message for an hour or more.
Besides adding this to the documentation, there should be 2 lines of code to check the first line of the file for those characters and report an error message to the user.
Chat support didn't have this documentation either. We only figured it out because I remembered it happening years ago, and we brute force tested uploading the file without the "#", then without the "/", then without the "-" until we had only the 26 letters of the alphabet, plus spaces, left.
This is such a basic thing (importing orders) I don't know why it hasn't come up and been identified, instead of saying "just use our column headers exactly" without knowing why that makes a difference when you're manually mapping columns.