When you assign a service to the service user, the account needs to be activated first so that the user will be created for the assigned service, you can then also set the password for the user for that service.
Click 'Users' followed by a click on the service user you created and assigned a service to
You will see one or more assigned services which possibly are not activated.
To activate click ‘Actions' > 'Activate’
Now choose a password and click 'Activate'
This will create the user account in the assigned service.