If role based pricing is enabled, does the user have the proper role that has a sell price attached?
Order status is 'Payment Received' instead of 'Completed.' Users are not enrolled.
Is the course marked as a shippable product? Reasoning: If a course is made to be shippable, the system expects administrators to switch order statuses from 'Payment Received' to 'Completed' once the product has been shipped. Setting is located in the pricing tab of the course.
Does the order have comments from the payment gateway and do they note a success or failure reason?
Was a configuration on the payment gateway changed recently?
User receives an error when trying to checkout.
Does the order in EthosCE have an order comment from the merchant/payment gateway? Please be sure to filter orders by <Any> to find orders set to 'In checkout' or 'Abandoned'. In most cases, a comment will be posted to the order from the merchant with further information (IE. billing address doesn't match, credit card type is not accepted, etc).
Is the payment gateway having issues? Please check your payment gateway's issue log/status page.