Lookup element on the Screen Flow can be used for User Lookup in Salesforce Flow.

Sample Flow:

In the above Flow, I have used “Field API Name” as CreatedById and “Object API Name” as Account. When the flow runs, it will allow me to search Users with this configuration.

When I tried with OwnerId insteaad of CreatedById, I was getting “Field: OwnerId is not a valid lookup field.” Exception.