Please see the following from directly inside the Stripe dashboard on the Customers page:
"BILLING
The Billing customer portal is now available
With the new customer portal, your customers can update payment methods, manage subscriptions, and view their payment history. Access to the customer portal is only available to paid Billing users. If you started using Billing before it became a paid product, an upgrade is required."
Additionally, this is from the Settings / Customer Portal page:
"Limited customer portal access
You can try out the customer portal in test mode, but full access to this feature is only available to paid Billing users."
Please let me know, thank you.