Why activated plugin does not appearing in left side menu bar of WordPress admin area?

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For Menu and Submenu,

enter image description here

function doctors_gallery(){
    add_menu_page(
        __( 'Doctors Gallery', 'textdomain' ),
        'Doctors Gallery',
        'manage_options', //  The capability required for this menu to be displayed to the user.
        'DoctorsGallery',
        'doc_gallery_view'
    );

    add_submenu_page(
        'DoctorsGallery',
        __( 'Doctors Submenu Page', 'textdomain' ),
        __( 'Doctors Submenu', 'textdomain' ),
        'manage_options',
        'DoctorsSubGallery',
        'doctor_submenu_callback'
    );

}

add_action( 'admin_menu','doctors_gallery' );

function doc_gallery_view(){
    echo "ghfhgfgh";
}

function doctor_submenu_callback(){
    echo "Sub Menu section";
}