"International and Self-Billing Payables Invoices - Outbound" "External Partner not found"

Summary:

we got inbound invoice working but we now want to start using self-billing invoice functionality, so am configuring "International and Self-Billing Payables Invoices - Outbound" via CMK and "External Partner not found" with a service provider in the mix for Belgium

As part of 26B turned on "simplified messaging" in order to convert the supplier already setup , as messages were enabled for override and can't use normal screens, only redwood (from what i know)

Trying to just get email configuration as step 1

have managed to get the message to CMK via "Payables Invoice and Life Cycle Status Extract for France“ for the invoice and xml looks as expected, but tests are falling over with "External Partner not found"