If the mail merge document was created by cutting and pasting:

    • Open the document for editing
    • When prompted by the error,  select "Remove Field" until the prompt stops to remove each field.
    • Close the document (You can use the keyboard shortcut Alt + F4 to close the document and Ctrl + W to close the entire Microsoft window) and click Yes when prompted to save
    • Open the document for editing again and you will get the standard Word screen.
    • Add the fields back an you normally would.

      If this is still an issue:

      Recreate the mail parameter file.