Hi,
As per moodle docs, here are the possible reasons related to CSV file format to cause the error:
- First line must contain column headings SPELLED correctly
- There should not be quotes (i.e. ")
- There should not be a trailing comma (i.e. ,)
- When uploading custom profile field names, make sure that the short name is in lowercase
- Usernames must be in lowercase in the file to be uploaded
Please check above and try again. If still the problem persists, would you please provide the CSV file. So that it is possible to verify the records and let you know further suggestions.