Before I go on, I have no experience with mergerfs/snapraid so my response is based on about 10 minutes of googling. Hopefully it will be of some value, but I'm not 100% sure. It's perhaps also worth mentioning that TurnKey is based on Debian. So anything relating to Debian, also applies to TurnKey. Ubuntu is often close (although note that it's not binary compatible with Debian - TurnKey is).

You mention "nextcloud container" so I'm guessing that perhaps you are running TurnKey Nextcloud as a LXC container on Proxmox?

If you are running Nextcloud so some sort of container, the "permission denied" error suggests that to work, it is being blocked by the container security measures.

If I'm on the right track, then I suspect that the solution will be to either to grant required permissions to the container. E.g. perhaps try enabling "nesting"? Otherwise, perhaps try a privileged container - although please be aware that without nesting enabled, many services will likely fail, but enabling nesting on a privileged container is a security risk.

So you might be better off running Nextcloud in a "proper" VM? If you want to try that, you can download the ISO from the Nextcloud appliance page.

I hope that helps. Please post back whether you need more of a hand or work it out yourself.