Do you notice an Error Message – Please select a Simple, Virtual and Free product.
To resolve this issue, you will need to select Simple & Virtual Products.
The Optin Step type only allows selling the above-mentioned type of products. This Step Type provides only three fields such as First Name, Last Name, Email ID.
So the user cannot make a payment in this scenario, this is the reason you need to set the zero price value Simple / Virtual product.
If you need to add more custom fields in the Optin Type you will need the CartFlows Pro version.