The reason you are unable to export the insight report is because, in order to export the insight reports to an excel format, the menu option CorePortal - Insight Hide Export Report Data (MENU ID: MAIN_MENU.ANALYTICS_EXPORT_REPORT_DATA) needs to be disabled from each respective menu profile that is assigned to users.
This can prevent you from exporting cases even in circumstances where other menu options are configured properly. To assign menu options to a menu profile, follow the steps on the article.