Update:
Even after conversion to a shapefile and importing the point layer into Arcgis online, I still can't configure the pop-up window. When clicking on Configure Pop-up nothing happens except a javascript message (see attached photo). I edited everything in the file that I believe could have been causing issues (strange characters in header row, etc.) but nothing seems to help. I'm stumped.
Edit: After multiple page refreshes and map saves it finally let me configure the pop-up for the shape file points, but it isn't consistent. Sometimes I'll get the javascript message and nothing else happens, and other times it will open the Configure Pop-up menu window. Very bizarre behavior. I still get the attached error for any text file I upload, however.