Elias here with Shopify Support. Thank you for reaching out.
I understand you’re running into an error that indicates the payments is not available for your shop. Just to clarify, the development plan does not include an active checkout so you wouldn’t be able to use the test mode with Shopify Payments. Once you or your client has decided a suitable plan to move forward with, you’ll be able to begin testing your payments.
Have you or your client get a chance to check out our monthly plans?