I do not know any Turnkey, so perhaps they have some specific documentation about what to do after installing.

The first time I installed Nextcloud, I did it on bare metal in Debian, following the LearnLinuxTV guide. It went through things like how to configure the Opcache, which is one of the warnings you have. If you’re not following a guide, you’ll have to find one, or search each message individually and research a solution.

Eventually I decided to reinstall with NextcloudPi because they know a lot more about systems administration than me, and had sane defaults set for most things. My first installation had some bugs that were probably from default configurations or mistakes I made perhaps. But I’ve had a good experience with ncp.

One last thing you’ll learn while researching the messages: some may be from bugs in the software that will be solved over time with nextcloud updates. Some will have clear solutions. Some may be generalized messages arising from network instability (like if Nextcloud is trying to sync over a weak Wi-Fi signal sometimes) or a hardware problem that can be tricky to track down, but might not be important. Just make sure you are doing good backups and you’ll be fine.