The error occurs when a begin or end merge field is missing from the acknowledgement message. Certain merge fields can only be inserted in between a begin section and an end section merge field. Insert the merge field being referenced in the error into the acknowledgment message by following the steps below.
  1. Edit the acknowledgement message
  2. Click the Insert tab
  3. Click the Merge Fields button
  4. Find the appropriate merge field, that is exactly what is mentioned in the error message 
  5. Drag and Drop the merge field into the message in the appropriate place
  6. Click Save for the acknowledgment message

Note: When you view the merge fields list, the merge fields that have to be between the begin and end merge fields will be listed that way. 

For example: 
The individual merge fields show listed in the following order in the merge fields list. 

Individual.Begin Section
Title
First Name
Last Name
Individual.End Section

Therefore, the merge fields for Title, First Name, and Last Name must be in between the merge fields for Individual.Begin Section and Individual.End Section