In case you want to setup a workflow configuration in the app "Manage Workflows for Supplier Invoices" and you want to setup specific users as approvers, those users must satisfy specific requirements before they are visible in the value help (see screenshot below).
Following requirements are necessary for a user to be selectable via this value help:
- The user needs authorization to the My Inbox app, otherwise they would not be able to receive the work items.
- The user needs display, change or create authorizations for supplier invoices.
- The user is currently or soon-to-be employed in the company.
Those requirements are applicable for both flexible workflows for supplier invoices ("Workflow for Blocked Invoice" and "Workflow for Parked Invoice as Complete").
For more information about the flexible workflow for supplier invoices in general please visit the SAP Help Portal https://help.sap.com
This blog is also valid since S/4HANA OnPremise 1909.