To disable the checkout form validation error, make changes to this file main/controllers/Checkout.php on line 520
replace
[php]$this->form_validation->set_rules('address_id', 'lang:label_address', 'xss_clean|trim|integer|callback__validate_address');[/php]
with
[php]$this->form_validation->set_rules('address_id', 'lang:label_address', 'xss_clean|trim|integer');[/php]