In the receiving Plant Customer has to be assigned in Assign Shipping data for Plants and also for Vendor Master has to be assigned to that Customer.
In addition, Vendor Excise and Customer Excise details has to be maintained in J1ID transaction with complete Excise details. If any of the details are not known you can use Hyphen in place. I dont think Supplying Plant needs to be assigned in Vendor Master record as it is needed only for Inter company STO.