your function name is UIIFS so that should be your output -- presently UIIFS has no value assigned to it hence 0 return

one further note, VBA is (unlike native XL) case sensitive by default -- i.e. TacoS <> Tacos... if you want to mimic IFS then consider convert both Criteria and Source strings to common case

Select Case UCase(Client)
    Case Is "TACOS"
        UIIFS = "Tacos"
...
End Select