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Sure you just need to have a different supplier product code (tab Suppliers, field ITMREFBPS) for each product record. By default the value is empty. So the system forbid you to have 2 empty supplier product code for the same supplier. The workaroud is easy, you need to enter a value, if you have none then you can use the Product code (ITMREF) to feed the supplier product code.
To make simple, the field highlighted in red should not be empty. Please enter a value and ensure there is no duplicate for the same supplier. By default enter the product code (ITMREF).