The IIF statements are missing the false part for it to evaluate.

IIF(<test - evaluates to true or false>, <what to do if true>, <what to do if false>)

=iif(

Parameters!GROUPING.Value = "Staff", Fields!UserCode.Value

, iif(

Parameters!GROUPING.Value = "Client ID", Fields!ClientID.Value

, Fields!ProgramName.Value

),""

)

Add something at the end to tell the expression what to do if it is false.