Hi Hardianti ,
Thank you for contacting OpenLearning Customer Success!
I'm afraid you can't actually delete the Modules completely from the course.
However, rest assured the pages/modules that are on the Additional Pages column are not visible and accessible to students so it shouldn't interfere with the learning modules that you have put up on the navigation link.
This is designed as such so that the modules still exist in the course in case you ever need it in the future.
Hope this information helped!