Thanks for the answer, I am a little confused about hooks, can you explain more how can I unset the error.

BTW, I tried to comment all this command in all the files I found $this->error['postcode'] but it is still mandatory.

if ((mb_strlen($data['postcode']) < 3) || (mb_strlen($data['postcode']) > 128)) {
     ////$this->error['postcode'] = $this->language->get('error_postcode');   
 }

See in the attached pic where is the field appearing.

Thanks!