This strictly only happens when the price is zero? Have you tried any other promotions or without promotions and it works fine? Is this with a plan that's single payments only, recurring only, or allows both? Typically a single payment basket made free with a promotion should immediately activate when clicking the button, which should've changed to a "Confirm Order" button. Sounds like you're requiring users to accept the terms and conditions before paying though. Try turning that off within CBSubs > Settings > Display > Baskets and see if your issue persists.

Strange, working fine in my tests. Please PM backend super administrator login credentials as well as exact steps to reproduce the issue and will take a look.

Have reviewed your install. You're using a custom CBSubs template, which is where the problem is. The button isn't type submit in your template which is why clicking it does nothing. Switching back to default CBSubs template resolves your issue. Recommend comparing your custom template with default and updating it where needed.