This error will occur on CSV files sporadically.  Here are two common causes for this error along with what you can do to resolve the issue:
  1. The commas exist, but are not appearing in the Excel window - Open the CSV file in a text editor (like Notepad).  Check to see if there are two commas at the end of the row or if there is a blank column header.  If so, you can remedy this by selecting the blank columns at the right of the spreadsheet in Excel and deleting them to be sure there isn't a column of data that is blank.  Save and attempt to run the upload again.
  2. There are additional field columns in your CSV file - Make sure that your file only includes the following two columns of data: PRIMARY_EMAIL and EXT_GROUP_ID, in that order.  Remove the additional columns, save and attempt to run the upload again.  If you need to make updates to other constituent record fields (name, email address, etc), you will need to do so through the Custom Constituent Data Import process.