Customize WooCommerce Error Message

You can do this with the woocommerce_add_error filter. Add the following to your functions.php file.

// alter the subscriptions error
function my_woocommerce_add_error( $error ) {
    if( 'The generic error message' == $error ) {
        $error="The shiny brand new error message";
    }
    return $error;
}
add_filter( 'woocommerce_add_error', 'my_woocommerce_add_error' );