function mandatory_product_fields(){
  echo "<script>
        document.addEventListener('DOMContentLoaded', function(event) {
            if(window.location.href.includes('post-new.php?post_type=product')){
               document.getElementById('post').onsubmit = function(){
                 var currentThumbnailId = document.getElementById('_thumbnail_id').value;
                 if(currentThumbnailId == -1){
                     alert('Please set product image.');
                     return false;
                 }
               }
            }
        });
  </script>";
}
mandatory_product_fields();