Remove the 'Billing' word from the error validation message in WooCommerce. When using WooCommerce, the checkout billing validation has the word 'Billing' automatically prefixed. Use the following filter to remove this.

remove-billing-word-from-validation-message.php

  add_filter( 'woocommerce_add_error', 'woocommerceAddError' );

   /**
     * Remove billing from the validation message
     * @see wc_add_notice
     *
     * @param $error
     * @return string|string[]
     */
    function woocommerceAddError ( $error )
    {
        if ( strpos( $error, 'Billing ' ) !== false ) {
            $error = str_replace( "Billing ", "", $error );
        }
        if ( strpos( $error, 'Facturering ' ) !== false ) {
            $error = str_replace( "Facturering ", "", $error );
        }
        return $error;
    }