This error is occurring when the Payment Txn object is trying to be updated.

The update is normally part of the Batch Processor that runs every 10 minutes.  If the update fail, then the updates will be retried again 10 minutes later, so you may get the same error continually sent.

Some common errors to look out for/check are:

• Error: Payment Form is incorrect for this Merchant Facility., Field AAkPay__Payment_Type__c
  
  This error message means a data issue on the Payment Txn.  It is saying that the Payment Form (api name of AAkPay__Payment_Type__c) does NOT belong to the Merchant Facility.  You may need to manually update the Payment Form and save.
  This could occur as a result of one or more of the following:
    -  you edited the Payment Form and relinked it to another Merchant Facility
    -  the URL Parameters for the checkout form have a Payment Form record Id passed in that does not match the Merchant Facility
    -  a URL Token was used and the Payment Form on this does not belong to the Merchant Facilit
    -  the Payment Txn is a recurring Payment and the related Recurring Payment - Payment Form on this does not belong to the Merchant Facility
• Error: A message saying some data is not entered correctly.
  
  This could be a result of a Validation Rule that you have added to a Payment Txn, Contact, Account, Contact, Opportunity.  Use the Object Manager to check each object and look for the Validation Rules error messages to see if they match the ones below.  Validation Rules are NOT covered by our standard support - you will need Premium Support if you need further help.  Please also review What are the criteria that must be included while creating Validation Rules on Payment Txn Object? | Payments2Us - Frequently Asked Questions | Payments2Us Manuals