Your theme has this functionality.
However, there are 2 problems:
First, in your assets/shop.js find function doAjaxAddToCart and inside this function there is this code:
$(".ajax-error-message").text(e.parseJSON(n.responseText).description);
this should be
$(".ajax-error-message").text($.parseJSON(n.responseText).description);
e → $
This should bring your error message box back.
However, contents of the box are white text on white background – probably something you can change in the theme settings.
Or can add the following to the very bottom of your frame.scss(.liquid ?) asset:
.ajax-error-modal .modal-inner {
color: red;
}