The script creates a report to show you which placeholders exists in InDesign document layout but not in the Data Merge source file.
Open your InDesign document with the placeholders, select the source file in the Data Merge panel and run the script. A txt report (unsaved) will be be open showing you the Data Merge Field Name, the document Placeholder text and the Location (page or pasteboard). Save the report and open it in Excel, for example, to better visualization. Easy to help you to find problems with DataMerge.