On my previous tutorial, you can read how to install and configure Nextcloud 12.0 on Debian 9 server. On a basic installation, you can access your Nextcloud server using http protocol. But in order to improve security, it is recommended to enable the SSL feature so you can access the Nextcloud via HTTPS. To enable SSL or HTTPS on Nextcloud is very easy.
Steps to enable SSL on Nextcloud 12.0
On Terminal, simply execute the following commands
a2enmod ssl a2ensite default-ssl service apache2 reload
Now you can access Nextcloud using https. Thank you