When you add a new campaign variation to any campaign, you have the option to use the visual editor. In order to use it, you have to choose one template to use.
If there is only one template option, then it means you don’t have a customized template. For more details click here.
After choosing the template you want to use in the editor, it will show in the working space.
For more details about how to edit a campaign in the visual editor click here.