Inside controller : 
return back()->withError('You are not logged in or Your session has expired');

Inside blade file : 
 @if(Session::has('errors'))
     toastr.error("{{Session::get('errors')->first()}}");
 @endif
   
It’s a “magic” method, in that any call to a method with a prefix of with will 
be added to the session as flash data. So withError() will add flash data
under the key error; withErrors() will add flash data under the key errors; 
withSuccess() will add flash data under the key of success; and so on.