Checklist Condition

How do you add a condition to checklist that users have to make only one selection?

I'm assuming you're working on this form https://www.jotform.com/63535635892163

The Checklist Widget is like a revamped/customized Checkbox. And checkboxes are meant to allow multiple selection.

If you need to limit the selection to just 1, you should be using a Radio Button instead (or widgets that are meant to act like a Radio Button).