During your sync operation, you receive an error message showing data validation failed BP Code assigned to BP, enter a unique BP Code 
on target application, which is SAPB1 here.

Cause

• Business Partner Code “XXX” already assigned to BP 
• When Multiple website/application is running in the same company (SAPB1). E.g: 
• Duplicate data is mapped for the BP CardCode.

Resolution

• Suppose you are syncing data from two different application into SAPB1, where cardcode value format is same in both the mapping, then it will create error while posting data in SAPB1. Change integration mapping  to specify unique Card Code for each BP entity, even after the error persist then use duplicate data check.
• Use duplicate data check, while you get error in a single application. Refer on How to use duplicate data check in data mapping