This error may appear if you edit the name of your Full Name merge field. The name of this merge field must be aliased as "Full name".

To edit the alias of the merge field: 
  1. Click Marketing and Communications > Appeal Mailings
  2. Click on the name of your appeal mailing
  3. Click the green dropdown arrow next to any letter > click edit
  4. Click edit under the Select receipt information to use to personalize your letter section next to merge fields
  5. Click on the Full Name field and click on the Pencil Icon above the Selected Fields Column
  6. Type in "Full name" without the quotation marks
  7. Click Save and Close
  8. Click next > Click Finish and Close