@klorimer, yes in the later updates Sales Part Cross Reference is imposed in several flows. I have come across few in Incoming CO flows. What is the context of your flow?
In one situation, we opted to use GTINs only in the incoming message, where it helped to function without using the Sales Part Cross Reference.  

@Vimukthi Mahakumbura thank you for your fast response.   This is a bit of a pain that for every sales part we will need to have a cross reference per customer ?  Why would this not have remained optional?  We do not require both Customer Part No. and a Sales Part No. in the incoming message as both are the same.  

@Vimukthi Mahakumbura sorry I  should have stated that it is in the Incoming Customer Order we are getting the error.

@klorimer, yes I agree with you, it can be cumbersome. But that is required basic data. What I hear from RnD is that, there have been several developments in this area, and that has caused this change. 
May I know what information is passed towards your ICO, with regards to the part no information? 

Hi @klorimer ,

This part of the code was introduced from bug id 150878.

If the Sales_Part_Cross_Reference is not entered, no internal PO is used and If the customer part number has a value, the code will raise the above error message.

According to the code , below are the ways of bypassing this error.
1. If the customer part number is null
2. If an internal PO is used ( when the sales part is available in both sites, customer_part_no can be used in place of catalog_no, even though Sales_Part_Cross_Reference is not entered) 

Best Regards,
Oshada

Thank you @Oshada Samarasinghe we will try and use your option 1 , leave the customer part number null. We just need to understand which field to update for the Sales Part No. and not the Customer Part No. 

The below is the current setup Customer Part No. , unsure of which line needs updated for the Sales Part No.