Enable mod_proxy_http module in Apache
In Apache of Ubuntu, the mod_proxy_http module that provides the proxy function by HTTP / HTTPS is not enabled by default, so enable it.
sudo a2enmod proxy_http
Please Restart Apache for the settings to take effect.
Please refer to the following articles for how to enable / disable Apache modules.
Note that mod_proxy_http depends on mod_proxy.
Apache error message when mod_proxy_http does not exist
Example Apache error message when mod_proxy_http does not exist.
Invalid command'<Proxy', perhaps misspelled or defined by a module not included in the server configuration