Here are some steps to follow and check when you encounter this error after trying to delete a disabled ''Campaign''
CAUTION BEFORE PROCEEDING: Any rule or campaign deleted will not and cannot be recovered.
Step 1. Go to the gear wheel icon, Select the Rules tab, then go to Campaign Rules and look for the ''Campaign'' rule that you might have associated with the existing ''campaign'' you wanted to delete. As soon you have found this rule, Click on the small gear wheel icon beside it and make sure to Delete. Then, Click Save.
(If there's no rule associated with the ''Campaign'' then you can disregard this step and proceed to the next step below)
Step 2. Open the ''Campaign'' and go to the Add to these Signatures area. Make sure that you remove ALL the Check Marks you can see associated with the ''Campaign'' you are trying to delete. Click Done to save.
You can now try again and delete the ''Campaign'' that you were unable to do so earlier and you should now be able to delete it.