Just try deleting that call and creating it again.
The id of the user for the reminder is not being successfully retrieved from the database. This might be a transient error, caused by some changes to your own user, or by some other previous error when creating the call.
Anyway it would be nice to know that the workflow runs correctly if that problematic call doesn’t exist at all. Later you can try re-creating it and diagnosing what else needs to be fixed