This can be caused by a registration that does not include a required field.  Check through your existing registrations for any record that is missing a mandatory field like email, first name, last name or any other required field.  Updating the constituent record of the participant will fix the issue.
  1. Constituent360 > constituents
  2. Search for the constituent
  3. Click Edit next to the record
  4. Populate the required field marked with * symbol