You are going to have to provide more information.
- How/where are you deploying it
- What web server are you using, NGINX, Apache, IIS?
It is very likely due to a misconfiguration of your web server. All URLs need to be rewritten to your index.html file. How you do this depends on the web server. With Apache, the .htaccess file. With NGINX, the site’s *.conf file.
You should be able to search for “rewrite to index.html” with the web server you are using and find the needed syntax/configuration.