Hello Dear,

You can set menu like this: 

menuitem id="menu_id" name="Menu_name"  web_icon="module_name,static/description/icon.png"/>

If your syntax is perfect you can check these points also.

--> Make sure odoo has proper access to your icon file: static/description/icon.png (In case if odoo user do not have proper access to file it will not work)

-->If menu is not visible it self it is possible that you do not have given access right to your custom module. (Add access in ir.model.access.csv)

--> If new in development make sure you updated module. (To be sure uninstall and install module again : Don't do it in production)