Are you getting this JavaScript error while clicking Complete Purchase button?
An invalid form control with name='card_name' is not focusable.
I was getting this this error after user applies their promo code.
In order to fix this just add this to your functions.php file and you should be good.
<?php
/**
* Plugin Name: Easy Digital Downloads Discount Patch
*/
add_action( 'wp_footer', function() {
?>
<script>
jQuery( function() {
jQuery( document.body ).on( 'edd_discount_applied', function() {
window.location.reload();
} );
} );
</script>
<?php
} );
Let me know if you have any question on this.