One reason the upload could be failing is if you are using a csv template that does not match the template provided to you by your Learning.net PM. There is an example of the csv format on the Import Users page.  The csv template is specifically formatted to work with the Import Users feature.

Another reason that could be causing the error is if the csv template includes any special characters (Such as dashes, hyphens, apostrophes, tilde, acute, etc).  Please be sure to remove any of these special characters before uploading the csv file.