Xamarin forms button not disabling after giving command binding

Hi

I am using this below to disable the button when the page opens

But the button is disabling when i am giving Command .

Please suggest what to do.

My Requirement is
I am trying to create an login page with viewmodel binding.
Having 2 entries and button.
At first i want to disable the button, when the 2 entries has validate the respective behaviours ,i want tot enable the button.
but the button is not disabling when the page loads ,when binding the command to viewmodel ICommand.