无法将任何控件/设置添加到nav_menus 因为那不是一个部分，那是一个面板。和控制/设置只能添加到节中。因此，您必须首先在nav_menus 面板，然后将控件/设置添加到该部分。检查以下代码

function nssra_customizer_options( $wp_customize ) {
    // add a custom section
    $wp_customize->add_section( \'nav_menus_custom\', array(
        \'title\' => __( \'Custom Section Title\', \'nssra\' ),
        \'panel\' => \'nav_menus\'
    ) );

    // add "menu primary flex" checkbox setting
    $wp_customize->add_setting( \'menu_primary_flex\', array(
        \'capability\'        => \'edit_theme_options\',
        \'default\'           => \'1\',
        \'sanitize_callback\' => \'nssra_sanitize_checkbox\',
    ) );

    // add \'menu primary flex\' checkbox control
    $wp_customize->add_control( \'menu_primary_flex\', array(
        \'label\'    => __( \'Stretch the primary menu to fill the available space.\', \'nssra\' ),
        \'section\'  => \'nav_menus_custom\',
        \'settings\' => \'menu_primary_flex\',
        \'std\'      => \'1\',
        \'type\'     => \'checkbox\',
        \'priority\' => 1,
    ));
}
add_action( \'customize_register\', \'nssra_customizer_options\' );

// sanitize checkbox fields
function nssra_sanitize_checkbox( $input, $setting ) {
    return sanitize_key( $input ) === \'1\' ? 1 : 0;
}

Update

您必须添加节description 要创建类似于外观的菜单位置，如果要更改节位置，可以设置priority. priority => 5 将在菜单位置之前放置自定义部分，如果这样做，则可能需要调整一些样式。请检查下面的代码以获取更新的节配置。

$wp_customize->add_section( \'nav_menus_custom\', array(
    \'title\'       => __( \'Custom Section Title\', \'nssra\' ),
    \'panel\'       => \'nav_menus\',
    \'description\' => __( \'Section description goes here.\', \'nssra\' ),
    \'priority\'    => 5
) );