You may need to delete a template group before adding another if you're in the Business tier, which has a maximum number of five template groups. To delete a group, go to Template Settings in the Admin app > Template Groups, then click Delete for the group you no longer need.
If a template is included in a group that's deleted, it won't display for users. You can quickly identify templates without template groups from the Manage Templates tab. A red exclamation mark displays on the tab if there are templates without template groups.