The clearance is the difference between the Attach (the distance between the ground and the cable node) and Sag:
If the report shows N/A in the Clearance column it means that the Attach is equal with Sag or the Attach is not indicated. The clearance should be automatically calculated since the program calculates the Sag value. You don't need to input any additional information. I suggest you contact the developer for further information.
Note: I tested the latest version of the application on my computer and the report is not displaying any Clearance column: