Thank you all. I have done and it works

Please follow Rafael first method in custom js

'm sorry Peter, you are right!
I'm so much used to working with my GreenByGreen theme which doesn't have this restriction at the admin backend that I forgot.

Thanks for the reminder, I had just remembered the function!

Interestingly though, when I put inline script on the button on the page, that script was stripped out on save. Indeed, when one puts in script tags, they are also stripped from the page (as they are here!). Obviously a security feature.

Add $(document).ready(function () ... in your JS code should be make your button clickable

$(document).ready(function () {
    $("#butt1").click(function(){
       alert("The paragraph was clicked.");
    });
});

Or use the old way

<p><button id="butt1" onclick=" alert('The paragraph was clicked.');">Go Back</button></p>

Yes.
Open your browser's developer console and you'll see that custom ccs/js is placing your script inside of the head section while your button code appears a lot of lines later.
In other words: You're trying to address an element that's still not there.
So why don't you put your script BELOW the button code on your custom page?