A: The most likely reason is because this open credit or payment has been allocated to a sales order. Refer to the following field in your Global Setup for the Order Billing Screen:
4. Use Payment Window for Real Time Cash Receipt
If this flag is set to "Y," then the user will be able to apply open credit or payment on the customer account to a sales order. The idea is to reduce the balance or pay the order amount in full. Then, during COP invoice posting, the open credit or payment will be automatically reapplied to the posted invoice.
In the above scenario, we have to protect the open credit or payment from being reapplied in the A/R module. To accomplish this, we added the field "Applied Amount" (AR_OPN_APPLY_AMT) to the A/R Open Item Table (AROPNFIL). If this amount equals the open amount, then it means this open credit or payment is already fully allocated to a COP sales order and, therefore, can't be reapplied in the A/R module.
Unfortunately, this "No Open Item On File" message doesn't really give a clear indication of what the situation is. In addition, the "Applied Amount" is pretty much a hidden field that does not show up in the A/R Customer Account Inquiry or the Aging Report. The only place you can see this amount is to go to A/R -> Maintenance -> A/R Open Item File. See the example below:
On 9/3/2014, we made a programming revision to the A/R Reapply function so if you should choose to reapply the open credit or payment record that's fully allocated, then we will display the message "This Credit Is Allocated to Sales Order" or "This Payment Is Allocated to Sales Order" to make the message a more clear reflection of the situation.
This update will be available in the next Elliott general release.