dont allow PO BOX shipping woocommerce not working

add_action('woocommerce_after_checkout_validation', 'deny_pobox_postcode');

function deny_pobox_postcode( $posted ) {
 global $woocommerce;

 $address  = ( isset( $posted['shipping_address_1'] ) ) ?          $posted['shipping_address_1'] : $posted['billing_address_1'];
 $postcode = ( isset( $posted['shipping_postcode'] ) ) ?    $posted['shipping_postcode'] : $posted['billing_postcode'];

$replace  = array(" ", ".", ",");
$address  = strtolower( str_replace( $replace, '', $address ) );
$postcode = strtolower( str_replace( $replace, '', $postcode ) );

if ( strstr( $address, 'pobox' ) || strstr( $postcode, 'pobox' ) ) {
  $woocommerce->add_error( "Sorry, we don't ship to PO BOX addresses." );
 }
}

i am getting:

fatal error :call to undefined add_error

when i am pasting on my function .php

 add_action('woocommerce_after_checkout_validation', 'deny_pobox_postcode');

function deny_pobox_postcode( $posted ) {
 global $woocommerce;

  $address  = ( isset( $posted['shipping_address_1'] ) ) ?     
 $posted['shipping_address_1'] : $posted['billing_address_1'];

 $postcode = ( isset( $posted['shipping_postcode'] ) ) ?  
 $posted['shipping_postcode'] : $posted['billing_postcode'];

 $replace  = array(" ", ".", ",");
 $address  = strtolower( str_replace( $replace, '', $address ) );
 $postcode = strtolower( str_replace( $replace, '', $postcode ) );

 if ( strstr( $address, 'pobox' ) || strstr( $postcode, 'pobox' ) ) {
   $notice = sprintf( __( '%1$sSorry, we dont ship to PO BOX addresses..' , 'error' ) , '<strong>' , '</strong>' );

        if ( version_compare( WC_VERSION, '2.3', '<' ) ) {
            $woocommerce->add_error( $notice );
        } else {
            wc_add_notice( $notice, 'error' );
        }
  }
}