This error indicates an invalid merge field has been added. Usually, as a result of copying content from one location to another. 

To resolve, remove the merge field(s) and insert them through the editor:
  1. Click Insert>Merge field
  2. Double click or drag/drop your desired merge field
  3. Click Save when finished

You may consider removing the merge field(s) from the original content prior to copying and pasting the HTML into another area.

Additional Resources
Cannot select or change the recipient list for an email message, or the merge field button is missing (includes video demo)