get_post_custom_values not working

It most probably means that no custom post values have been retrieved for the key Description that you used. According to the Codex, Returns nothing if no such key exists, or none is entered.

Try adding print_r($key_values); on the line just after get_post_custom_values("Description"); and check if any value is retrieved.

To skip this warning(when you don’t have any custom post values associated to a post), you should check if $key_values is an array before doing the foreach. And don’t forget to escape your double quotes.

if( is_array( $key_values ) ){
  foreach($key_values as $key => $value );
  echo  "$key  => $value(\"Description\") <br />"; #You should escape your double quotes here!
}