ExactJS library says - You are not permitted to access this service

I am using ExactJS and be able to render the form. On initiating the payment as per documentation, exactpay network call (https://api.exactpaysandbox.com/orders/4f494a32-4875-4304-8012-6d46f4e1fb7c/pay) has responded with error

messages: ["You are not permitted to access this service."]

What does this error mean? Where do we need to enable this service?