First, ensure only merge fields which are specific to that mailing are included in the content. For example, you are editing a donation confirmation, but copied and pasted a merge field from a registration confirmation email.

Next, convert the HTML to Text via:
  1. From the content of your mailing, click the Text Button
  2. Click the Convert HTML to text Hyperlink
  3. Click Submit in the popup window
  4. Click Save
If the error message persists, remove all the fields and re-add them, then convert HTML to Text. You may also attempt removing/adding one field at a time, though ensure HTML to Text conversion takes place after each edit.