Dover1104 Do you still need help? If so, is there anything on the lot you don't want to lose? For an empty lot, or one you can replace, you could bulldoze it in Edit Town and try again. Make sure there's at least one item on the lot so that bulldozing doesn't remove the lot itself.
If this doesn't help, let me know whether you use NRaas MasterController or would be willing to add it. You could click the ground of the lot, select NRaas > MC > Object Stats > Local and look for anything with "mailbox" in the name, i.e. some leftover data from the current one. Select that and agree to flush it, then try to add the cc mailbox.