A swimlane element contains other elements inside the lane. Before you delete the swimlane, you have to either move the elements outside the swimlane or delete the elements first; not doing so will result in a dependencies error.
For example, let’s say you want to delete this swimlane:
Delete the two Next outcomes and then the two Steps first. (You can also drag and drop these elements outside the swimlane.)
Click on the swimlane to select it
Click the trash can icon. This opens the Confirm Delete panel.
Like with all map elements, you can not undo the delete action.