Checkout Error with Free Product

I have a membership/subscription that is free in order to lock down pages based on those who have signed up for the specific membership (they are invoiced for other costs). However, I am getting an error when trying to check out and I can’t figure out what is causing it. Please help!

If you are testing it on my preview - you create an account and then select the “Drink Sponsor” Opportunity which will bring you to the product page to sign up.

Here is my site Read-Only: Webflow - Mission Coffee House Mosinee