There could be several reasons why the website dropdown menu is not working and not clickable in Odoo 11. Some possible causes include:

1. JavaScript errors: The dropdown menu may not be working due to JavaScript errors on the website. Check the browser's console for any JavaScript errors and fix them.

2. CSS issues: The dropdown menu may not be clickable due to CSS issues. Check the CSS styles for the dropdown menu and make sure that it is not being hidden or obscured by other elements on the website.

3. Missing dependencies: The dropdown menu may not be working due to missing dependencies. Make sure that all the necessary JavaScript and CSS files are loaded on the website.

4. User permissions: The user may not have the necessary permissions to access the backoffice. Check the user's access rights and make sure that they have the correct permissions.

5. Browser compatibility: The dropdown menu may not be working due to browser compatibility issues. Test the website on different browsers and make sure that it is working properly.

6. Third-party modules: Some third-party modules may be conflicting with the website and causing issues. Try disabling any recently installed modules and see if the issue is resolved.

You can start by checking the browser's console for any javascript errors, check the css and js files loaded by the website and make sure they are loaded correctly. Also, check the user permissions. If you can't find the issue, you can try disabling recently installed modules or try testing it on different browsers.