Business process error
The company on the selected Contact must match the company for the current Order record:
The contact does not have a Company in its record, but we can't add a Company to the contact - we get this:
Company: Company can be filled in only when 'Is Customer' or 'Is Vendor' is set to Yes
Of course, this is just an associated contact for the Customer record - it isn't a Customer or Vendor itself. The Customer has the same Company as the Order.