Hello, Federico
You can use the following transactions to link custom/vendor to BP:
FLBPD2 - Link Business Partner to Customer
FLBPC2 - Link Business Partner to Vendor
You can find them here: SAP Menu -> Accounting -> Financial Supply Chain Management -> Treasury and Risk Management -> Business Partners -> Special Functions -> ...
First link customer as customer is standard to whom we pay. That's why system issued the error to you.
Then link vendor to BP.
