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