Have you configured Elavon at X-Payments side? https://www.x-payments.com/help/X-Payments:Payment_configurations
Also, there are two Elavon integrations in X-Payments - Elavon Payment Gateway (new one) and Elavon via Virtual Merchant - Merchant Provided Form (old one). Which one do you need to use? You might be trying to use 1st Elavon account with the old integration or vice versa.