Stripe has two types of testing environments - legacy test mode and Sandboxes.

v2 endpoints only support in Sandboxes whereas v1 endpoints support both legacy test mode and Sandboxes.

To work on v2 endpoints, you should ensure the Sandbox is being set up and use its corresponding API keys.

Here are the differences between v1 and v2 namespaces: https://docs.stripe.com/api-v2-overview#key-differences-between-the-v1-and-v2-namespace