Old phone. Delete backup.

My iCloud has been full for years and unable to backup my phone. Recently my phone met its demise under several car tires. I now have a new replacement phone through insurance. I've activated the new phone and had to use the most recent backup to restore my phone (which is two years old). I have deactivated the old phone through the apple website and now Im trying to delete the backup as to free up storage space. I've purchased addition al iCloud storage So that I could backup the new phone and updated the iOS as well. I am unable to delete the old phones storage and backup. I keep getting a prompt that says unable to delet because that backup is being used. Help!