It's saying exactly what the error is - you're using the user() function and passing it a value Appian does not recognise as a valid User. Note that the value for a User is case sensitive e.g. Arunkh is not the same as arunkh. You can avoid this type of error by passing in the value in your rule input as the correct datatype i.e. set ri!userName as type 'User' and not 'Text'. Appian won't let you pick an invalid User for your unit test.
