Your screenshot shows a default 404 page from the webserver nginx.
In Mendix you can create custom error pages, like 404.html. Read more here: https://docs.mendix.com/howto/front-end/custom-error-page
Be aware of the pre-requirements. If you want to change this in your nginx, read this: https://ubiq.co/tech-blog/how-to-create-custom-404-page-in-nginx/
Please check the folder resources\deeplink in your project. You can use these files for a different 404 error message.
Although, currently you have a 404 from nginx, it looks like the 404.html is not in your resource folder or you are using a wrong url.