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.