Cannot instantiate interface DrupalStreamWrapperInterface
The error is because you are trying to create an object of a PHP Interface, which is not allowed in PHP. You can only create objects from a PHP Class.

To solve your issue, something like this might work:

$wrapper = file_stream_wrapper_get_instance_by_uri($img_path);
if (is_object($wrapper)) {
  $file_path = $wrapper-&gt;realpath();
}