Issue with Free Subscription in Membership plugin

On our website, I am using Membership plugin to create two different types of users. I have two access levels and two subscription plans. But we don't charge any money for subscriptions. User need to be register and don't need to pay anything.

On the first page of the registration, it asks to choose a subscription. One the second page, it asks for user login details for the registration. On the third page, it logs in the user, shows the coupon code page (as shown in the attached image). But nothing changes after that. There is no way to complete the process of registration and subscription.

Here is the image: