Error Pop up: An error occurred whilst submitting the checkout form [012]. Should this persist, please contact [Org name]. [Org name] PLEASE check error logs for more details.

Explanation:
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.
Error Log:
Program Area - payments2us.ValidateTransaction
Subject - Possible website fraud attempt [012], Merchant Facility Id: a0ZQE0000008tc12AA
Description - Maximum from an IP check has been exceeded - security settings have been automatically updated. Please stay vigilant and take other appropriate steps.
More security settings information is available at: https://help.payments2us.com/m/FAQ/c/413924.
If you are expecting multiple payments from a single source, eg. using an outsourced call centre, then you may wish to add their IP address to the IP Address Access related list on the Merchant Facility.
Alternatively, you can remove or update Max Submissions from Single IP on the Merchant Facility.
Solution:
Go to the Merchant facility->IP address section, you will see Blocked IP addresses. For the IP that is a legit transaction please edit and update the Access from 'blocked' to ‘allowed’. This will unblock those IP addresses.
See the link for Unblocking IP addresses. Merchant Facility FAQ 11.