This error is caused by an invalid column name or missing column name in your import process. Follow the below steps to correct:
  1. Navigate to Administration>Import
  2. Click drop down next to Import and select Edit
  3. Click Map fields tab and verify the Import file columns are as follows: Lookup ID, Last Name and First Name
  4. Save changes and Start import process once more
If issue continues, please Click Chat with Support and reference this article