Hi,

Please check the following steps:

1. Module Folder Location

Your module folder should be inside a directory that Odoo knows (e.g. custom_addons/ or addons/).

In odoo.conf, make sure you have:

addons_path = /path/to/odoo/addons,/path/to/custom_addons

2. Check __manifest__.py

Example:

{

    'name': 'Custom Sales',

    'version': '1.0',

    'category': 'Sales',

    'summary': 'Customizations for Sales',

    'depends': ['sale'],

    'data': [

        'views/sales_view.xml',

    ],

    'installable': True,

    'application': True,

    'license': 'LGPL-3',

}

- If installable is missing or set to False, Odoo hides the module.

3. Restart Odoo Service

After adding the module, restart your Odoo server

4. Update Apps List Properly

Go to Apps → Update Apps List → Update.

5. Remove "Apps" Filter

In Apps search, Odoo often applies the Apps filter (only official apps). Click the “Apps” filter tag to remove it, then search again.

Hope it heps.

Ensure your custom addon resides in a path that is actually part of your addons_path in the odoo.conf file. If the configuration file has changed, restart the server. 
Also, check the log whether the correct paths are loaded upon starting the server.

Finally, ensure the module is in the correct structure:

/x/y/z/custom_addon/__manifest__.py

/x/y/z/custom_addon/__init__.py

'/x/y/z' must be part of your addons_path in the configuration, custom_addon is the custom addon (folder) and in there at least a  __manifest__.py file must be present. For odoo.sh also a __init__.py must be present.