Why I do not see the "default" option?
This guide will teach how to find the Default option of the checklist templates.
If you open the global Checklists configuration for the templates in Administration (on top) - Plugins - Checklists - Checklist templates:
Then there is no Default option. It is by design of the plugin.
However, if you open the project-level settings of the Checklists, then the situation is a bit different. Please select a specific project, then Settings - Checklist templates.
The Default option is now appearing for your Checklist template.