Hello, @jakezmarkyt. Thanks for reaching out.
Thank you for providing us with access to your app.
Indeed, after the investigation of your issue, the root cause is in your customer data retrieve process.
The error you get has appeared because there is no information about a customer, so @johnny has a good point here. Stripe API unable to retrieve the customer data and display it within the Group Money Source (Group Billing source).
Please try to create a new app using the template and do not change anything in it (do not remove or add anything), except the Stripe setup. Add pay much of your attention to the subscription you are creating in your Stripe Dashboard, and try again 
In a case, if you have created a new template copy without making any modifications in it, except the Stripe setup, and still see this error - please add our support account as a collaborator of that “blank template copy”. We will take a look at it too.
Some useful references:
https://stripe.com/docs/api/customers/retrieve
Also, you can check for the errors in your Stripe dashboard - go to Developers > Logs tab.
Create new customer, find it in your Stripe dashboard by customer ID and check this customer’s account and Logs (to find any info that can reveal why the customer’s data can’t be retrieved on the app side).
Thank you for understanding and patience.
Regards,
Zeroqode Team