When a report is generated the server application first creates an ods file.
If extension attribute of the report is set to ‘pdf’ or any other format except ‘ods’, the application first creates an ods file and then uses LibreOffice in “headless” mode to convert the ods file to that format.
If LibreOffice is currently running on the server this conversion may not happen. You must close LibreOffice on the server for the conversion to take place.