Hi youta

You cannot do it like that, Odoo is not an PHP application or something similar, there is no root directory where you can put that html file in order to access it like that, but you have a lot of options to get that result.

- Use a reverse proxy(nginx, haproxy, caddy, ...) in front of your Odoo so you can map the url /file.html with the real file

or

- Install Odoo website in your database and create a new page to serve the content of that file by copy its content to the new page, normally if you name your page file.html the url will be /page/file.html

or

- Put your file.html in the static folder of any module and you will be able to access it like, /module_name/static/file.html

or

- Create a controller that serve the content of that file as response from /file.html, the content could be taken from the real file or better from a cached Odoo template in the Odoo way to make this kind of controllers