Make checkout fields required in Woocommerce checkout

Question

For some reason, all the fields in Billing Address are marked as optional - customers are leaving the billing address fields blank and then their payments are being rejected

Answer 1

What you can do if you don't find the guilty as explained on my comment is to use the following (using here a highest hook priority if some other code is already using those hooks):

add_filter( 'woocommerce_default_address_fields', 'customising_checkout_fields', 1000, 1 );
function customising_checkout_fields( $address_fields ) {
    $address_fields['first_name']['required'] = true;
    $address_fields['last_name']['required'] = true;
    $address_fields['company']['required'] = true;
    $address_fields['country']['required'] = true;
    $address_fields['city']['required'] = true;
    $address_fields['state']['required'] = true;
    $address_fields['postcode']['required'] = true;

    return $address_fields;
}

Code goes in function.php file of your active child theme (or active theme). tested and works.

For billing phone and email you can try

add_filter('woocommerce_billing_fields', 'custom_billing_fields', 1000, 1);
function custom_billing_fields( $fields ) {
    $fields['billing_email']['required'] = true;
    $fields['billing_phone']['required'] = true;

    return $fields;
}

or

add_filter('woocommerce_checkout_fields', 'custom_billing_fields', 1000, 1);
function custom_billing_fields( $fields ) {
    $fields['billing']['billing_email']['required'] = true;
    $fields['billing']['billing_phone']['required'] = true;

    return $fields;
}