How to prompt error when one check box is missing

Not sure where you want this error message to appear or when. Presumably you'd put it in the form callback (it would get a bit annoying if it was on the toggle field callback). I would assume you'd just check the number of toggles which were enabled, or check for the specific combinations you wish to allow or disallow (your title doesn't really match the description of what you want).

Andrew

Lemme clear you more. If i run above code- i get this forum, now

in this i want ,when am selecting only test3 , it should throw error via dialog box that test3 cannot select alone, test2 and test3 both should be selected.
How can we do that as u say Presumably you'd put it in the form callback (it would get a bit annoying if it was on the toggle field callback)

drake

toggle_Entry=hiCreateToggleField(
  ?name 'toggle_Entry
  ?choices list(list('aa "test1") list('bb "test2") list('cc "test3"))
  ?value list(nil nil nil) 
  ?callback list("CCFupdatePathEnter(hiGetCurrentForm())")
)

procedure(CCFupdatePathEnter(form)
  let((listOfStrings)
    listOfStrings=
      setof(elem 
        foreach(mapcar (toggle data) form->toggle_Entry->value 
          list("1" "2" "3") toggle && data)
        elem
      )
    if(listOfStrings then
      form->path_enter->value=strcat("use " buildString(listOfStrings ","))
    else
      form->path_enter->value=""
    )
  )
)

procedure(CCFvalidateToggleFields(form)
  if(form->toggle_Entry->value=='(nil nil t) then
    hiDisplayAppDBox(
      ?name 'CCFbadToggles
      ?dboxBanner "Please take SKILL course"
      ?dboxText "Cannot select only test3"
      ?dialogType hicErrorDialog
      ?dialogStyle 'modal
    )
    hiSetCallbackStatus(form nil)
  else
    hiSetCallbackStatus(form t)
  )
)

path_enter=hiCreateStringField(
  ?name 'path_enter
; commented out so can see what is going on
;  ?invisible t  
)

hiCreateLayoutForm(
  'ForumForm
  "Forum test"
  hiCreateFormLayout(
    'lay
    ?items list(toggle_Entry path_enter)
  )
  ?callback 'CCFvalidateToggleFields
  ; allows the form callback to veto the unmapping of the form
  ?unmapAfterCB t
)
hiDisplayForm(ForumForm)

Thanks Andrew,
I have enroll for skill training started learning :))