Prestashop module admin controller (page not found)

Question

I'm trying to figure out how to work with controllers in backoffice.
The prestashop version I'm using is 1.7.7.4.
I want to make a page in the backoffice.
I made a controller "MyModuleController" in "/modules/MyModule/controllers/admin/MyModuleController.php", but in the backoffice appears the message "The controller MyModuleController is missing or is invalid".
What am I doing wrong? Below there is the code I wrote:

/modules/MyModule/MyModule.php

<?php

class MyModule extends Module
{
    public function __construct()
    {

        $this->name = 'MyModule';
        $this->tab = 'administration';
        $this->version = '1.0.0';
        $this->author = 'Firstname Lastname';
        $this->need_instance = 0;
        $this->ps_versions_compliancy = [
            'min' => '1.6',
            'max' => _PS_VERSION_
        ];
        $this->bootstrap = true;

        parent::__construct();

        $this->displayName = $this->l('My module');
        $this->description = $this->l('Description of my module.');

        $this->confirmUninstall = $this->l('Are you sure you want to uninstall?');

    }

    public function install()
    {
         // Install Tabs
        $tab = new Tab();
        $tab->active = 1;
        $tab->class_name = "MyModuleController";
        $tab->module = 'mymodule';
        $tab->name = array();
        $tab->id_parent = (int)Tab::getIdFromClassName('SELL');
        $tab->position = 3;
        foreach ($lang as $l) {
            $tab->name[$l['id_lang']] = $this->l('Mon module');
        }

        $tab->add();

        parent::install();
    }

    public function uninstall()
    {
        // Uninstall Tabs
        $tab = new Tab((int)Tab::getIdFromClassName('Mymodule'));
        $tab->delete();

        // // Uninstall Module
        parent::uninstall();
    }
}

/modules/MyModule/controllers/admin/MyModuleController.php

<?php
class MyModuleController extends ModuleAdminController
{
    public function __construct()
    {
        parent::__construct();
        $this->bootstrap = true;
        $this->id_lang = $this->context->language->id;
        $this->default_form_language = $this->context->language->id;
    }

    public function initContent()
    {
        parent::initContent();
        return echo 'hello';
    } 
}
?>

Answer 1

Follow Prestashop default modules:

module ps_linklist:

$this->name = 'ps_linklist';
    $this->author = 'PrestaShop';
    $this->version = '3.2.0';
    $this->need_instance = 0;
    $this->tab = 'front_office_features';
    $this->tabs = [
        [
            'class_name' => 'AdminLinkWidget',
            'visible' => true,
            'name' => 'Link Widget',
            'parent_class_name' => 'AdminParentThemes',
        ],
    ];

your admin controller's name must started with "Admin"