Area: "XYZ", field: "XYZ" is mapped to the data source header "XYZ" but that the header can no longer be found in the data source.
"XYZ" will vary based upon your mapping name, the field that you are working with and the default value that you have selected.
If you are working with a spreadsheet, then you should ensure that your column headers have not changed since you built the template. If you are working with a direct data source (such as Eventbrite, MobileCause, Classy, Qgiv, etc.), the field name may have changed within the API.
You may also see this if you select a different set of Data Source Criteria than you used when you built the template (choosing the donations rather than event participations for example).
To resolve this, either select a file that has the same column headers, change your data source criteria or re-map that particular field in your template.