But now although the host has been added, when I go to Monitoring->Dashboard, it shows 1 Unknown(nextcloud is unknown) and 1 Available(zabbix is available). Why’s it happening? And how to fix this issue?
Seems more like a Zabbix question than a Nextcloud question, and I’m no expert on Zabbix. However I can maybe provide few thoughts that might help to point you in the right direction:
If you just add your Nextcloud server as generic host, I wouldn’t expect Zabbix to be able to monitor much, except maybe ping or HTTP status and the information provided from the status.php page, for which you still would have to manually create the respective queries, I guess.
There is also Nextcloud template for Zabbix: Nextcloud monitoring and integration with Zabbix
However, the Zabbix forum would probably be a better place for asking questions about how to use and configure their templates.
It sounds like you added the host but have not added any actual checks for it to perform.
A simple check would be a TCP 443 port check. This would tell you whether the web server is running and reachable on the network.
If you want to do something more advanced, you could install the Zabbix agent on the Nextcloud server and set up any OS-level monitoring you want.
And at the application level, in Nextcloud if you go to Settings > Administration > System, at the bottom there is a link to XML/JSON system information from Nextcloud. You could potentially set up parsing and checking if this data in Zabbix. That would be the ideal method.