Got it.I understood that user has asked you to show up "Completed Status in SLA"even when IsNull([Respondby] .Is it right ?
Please add the below condition and let me know If it is not working.
=If (([WO Status] InList("INPRG" ; "WRKDN" ; "BILLED" ; "APPLIED" ; "APPLYFOR" ; "CLOSE")) And ([Actstart]<=[Respondby] or IsNull([Respondby]) And [Worktype]= "RW") Then("Completed Status in SLA")
Thanks,
Kalyan.