Hello Lisa!
You mentioned that you currently have 1,200 subscribers, and you are under the free plan. I noticed on the screenshot below:

It says you need to upgrade your account, to be able to send emails. It is because you exceeded the subscriber limit of the free plan of 1k subscribers.
That is the possible reason why you could not receive an incentive email when you tested your form. And the sign-ups are unable to confirm their subscription.
I recommend contacting ConvertKit's support so they can check further here: help@convertkit.com29.