// Add a second password field to the checkout page in WC 3.x.
add_filter( 'woocommerce_checkout_fields', 'wc_add_confirm_password_checkout', 10, 1 );
function wc_add_confirm_password_checkout( $checkout_fields ) {
    if ( get_option( 'woocommerce_registration_generate_password' ) == 'no' ) {
        $checkout_fields['account']['account_password2'] = array(
                'type'              => 'password',
                'label'             => __( 'Confirm password', 'woocommerce' ),
                'required'          => true,
                'placeholder'       => _x( 'Confirm Password', 'placeholder', 'woocommerce' )
        );
    }

    return $checkout_fields;
}

// Check the password and confirm password fields match before allow checkout to proceed.
add_action( 'woocommerce_after_checkout_validation', 'wc_check_confirm_password_matches_checkout', 10, 2 );
function wc_check_confirm_password_matches_checkout( $posted ) {
    $checkout = WC()->checkout;
    if ( ! is_user_logged_in() && ( $checkout->must_create_account || ! empty( $posted['createaccount'] ) ) ) {
        if ( strcmp( $posted['account_password'], $posted['account_password2'] ) !== 0 ) {
            wc_add_notice( __( 'Passwords do not match.', 'woocommerce' ), 'error' );
        }
    }
}