How to have exception work on a node in process model encountering activity class parameter error

Hi All,

I'm trying to add a exception to one of my nodes when I'm creating new users, checking the condition if the username is taken or not. I'm currently trying to add this exception to the create new user smart service node,  When I create this exception it gives me an error of activity class parameter? is it saying that the value giving to check it doesn't exist ? I'm not sure.