Unlike FIS where you create an Invoice Receipt, in uSOURCE you can’t manually enter the invoice.
If a supplier emails you the invoice, ask them to resubmit it through uSOURCE (and cc: uSOURCE Help desk on the email). The invoice will then be paid automatically once the 2-way or 3-way match is met.
See also: How are my uSOURCE orders paid?