I recently tried extcloud too, right on your own version of Turris OS (and in fact I even opened a thread here Nextcloud from reForis - limitations, looking for feedback). I followed this guide Nextcloud server on Turris and it was super easy. I don’t remember activating anything by hand (yes, maybe there isn’t a nextcloud service), and the whole web service stack (php, mysql and perl) is installed and started automatically.