"Scripts - Background" (found in Menu under System Definition -> Scripts - Background) require no plugin activation, but it requires by default admin role (see here). You can open "Scripts - Background" by usage URL https://<your_instance_name>.service-now.com/sys.scripts.do or https://<your_instance_name>.service-now.com/nav_to.do?uri=%2Fsys.scripts.do. If you don't have admin role, but have script_fix_admin role then you can create new fix script with the code from my answer and execute it.