what does the error mean?

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.

user type is there as a data type for rule inputs in expression rule?

The username which you entered is not exist. please try to use another username.

username is the which we use to login into the environment?

then use user(loggedInUser(),"firstName")&" "&user(loggedInUser(),"lastName")

i wanted to when i can set a rule input of user type?

this is fine i understood

because in an expression rule the rule input can be of primitive data types, CDT and record types right?

if the username which you are sending is valid then you can use the code below.

can you show me the rule input part?