I am not able to make mandatory or required field through rule - isNullOrEmpty(HostName), I don't want to do from list level.
Please refer the screenshot.
What could be the reason. It is happening to all of the controls, only fields that are made required from list, are throwing an error.
it looks like you typed in HostName manually as a parameter of the function.
you have to select reference to the control from 'Named controls' tab in formula builder.