How to save WooCommerce checkout custom fields to user meta

Solution:

first, you should add your custom fields like this: (use woocommerce_checkout_fields filter)

function reigel_woocommerce_checkout_fields( $checkout_fields = array() ) {

    $checkout_fields['order']['date_of_birth'] = array(
        'type'          => 'text',
        'class'         => array('my-field-class form-row-wide'),
        'label'         => __('Date of Birth'),
        'placeholder'   => __('dd/mm/yyyy'),
        'required'      => true, 
        );

    return $checkout_fields;
}
add_filter( 'woocommerce_checkout_fields', 'reigel_woocommerce_checkout_fields' );

adding 'required' and setting it to true will have the same effect with how you check if this field is set or not. (your “Error checking”)

then in your woocommerce_checkout_update_user_meta, the first parameter is not $order_id but the $customer_id. You should know too that the second parameter is $posted. $posted contains the $_POST[] data. Including your custom fields if you did the code above.

function reigel_woocommerce_checkout_update_user_meta( $customer_id, $posted ) {
    if (isset($posted['date_of_birth'])) {
        $dob = sanitize_text_field( $posted['date_of_birth'] );
        update_user_meta( $customer_id, 'date_of_birth', $dob);
    }
}
add_action( 'woocommerce_checkout_update_user_meta', 'reigel_woocommerce_checkout_update_user_meta', 10, 2 );