This issue has appeared on checkout pages where the theme does not include jquery in the checkout. 

You will need to add additional code via “Settings/Checkout/Additional Content and Scripts” field.

Add the following code to make the “$” namespace available.

<script>
if (window.jQuery) {
$ = window.jQuery;
} else if (window.Checkout && window.Checkout.$) {
$ = window.Checkout.$;
}
</script>