A red Business ID field does not prevent sending the invoice to Purchase invoices through Link.
In the event of a foreign supplier, you should fill in the filed for example VAT number found on the invoice.
In cases where the supplier or the individual customer does not have a Business ID or a similar identifier, the Business ID field can be filled in for example with the supplier's/individual's name with lower case letters without special characters, all written in a row.
The Business ID field can not be left empty as the teachings and supplier data are saved behind the Business ID.