If you want to delete a drawing set that has drawings in it, you will need to first delete drawings in that set.
Steps
Navigate to your project's Drawings tool.
Click the Configure Settings icon.
Under 'Drawing Register Settings,' scroll to the 'Edit Drawing Sets' section.
Click the delete icon next to the empty drawing set you want to delete. Note: You can only delete empty drawing sets. If a set has at least one drawing, the delete icon will be greyed out and inactive.