Sets all Billing fields on WooCommerce checkout to required

wc-billing-fields-required.php

/**
 * Sets all WooCommerce billing fields to be required.
 */
function wc_require_billing_fields( $fields ) {
	$fields['billing_first_name']['required'] = true;
	$fields['billing_last_name']['required'] = true;
	$fields['billing_company']['required'] = true;
	$fields['billing_country']['required'] = true;
	$fields['billing_address_1']['required'] = true;
	$fields['billing_city']['required'] = true;
	$fields['billing_state']['required'] = true;
	$fields['billing_postcode']['required'] = true;
	$fields['billing_phone']['required'] = true;
	$fields['billing_email']['required'] = true;
    
	return $fields;
}
add_filter( 'woocommerce_billing_fields', 'wc_require_billing_fields' );