Why do we get the error- An error occurred whilst submitting the checkout form[012,011,...]?
This error occurs when a Payment attempt is made from a blocked IP address. You will have to unblock the IP address. There will be an Error log logged in "Error Log-Payments2Us" tab with instructions on the steps that needs to be done.
Go to the Merchant facility->IP address section, you will see Blocked IP addresses, please unblock those IP addresses.
See the link for Unblocking IP addresses. Merchant Facility FAQ 11.