The merge fields [Merge Field.Begin Section] and [Merge Field.End Section] are known as conditional merge fields. They define the beginning and the end of a section that will only appear when an end user enters information that falls within those conditional merge fields.  Referencing the example above:

If a transaction fee is not collected, than any information between those begin / end fields will not display in the email.

To resolve the error: 
  1. Open the editor to the section that is producing the error
  2. Drag and drop the merge fields directly between these placeholders - [Merge field.Begin Section] and [Merge field.End Section]
    For example: [Transaction.Fee Collected.Begin Section][Billing Information.FirstName][Transaction.Fee Collected.End Section]

Additional Notes
  • The available merge fields in the conditional merge field grouping (example Billing Information grouping, including "First Name", "Last Name", etc.) will only insert form data if a payment is collected. Therefore, these fields are required to appear between the begin / end section merge fields. If you attempt to place any conditional merge field groupings without placing them between begin / end section merge fields, you will see an error message similar to what is noted above.
 
  • If the merge field in the error message, is not actually within the content, it's possible there is an invalid merge field that may have been copied/pasted from a separate location type. For example, a registration merge field was copied into a donation merge field. To resolve this, either the HTML in working content needs to be copied/pasted into the desired location or the content needs to be rebuilt. For example, if the error is occurring for a donation form acknowledgement email, find an acknowledgement email where the content is working, copy its HTML, and paste it into the desired donation form acknowledgement email.
 
  • If the merge fields stated in the error are not present or adjusting them does not resolve the error, then look for merge fields that are placed in-between loops that don't belong there. Only fields listed between the begin loop field and the end loop fields in the merge field list should be placed between loops. You may be able to resolve the error by deleting the field, saving the part, reinsert the field, and save again. If it saves without error, then the part may work. However if the error still occurs, re-organize the merge fields in the email / eReceipt / confirmation so that only the correct fields are placed between begin loop and end loop fields.