I don't know, but if I could install NixOS without difficulty, it should be possible. I installed Ubuntu server on it at first and that also worked fine. No tweaking necessary at all. You just flash the standard x64 installer on a USB stick, plug it in, and install like you would on any PC (because it basically is a PC - it even has a working HDMI port).

Edit: Looks like someone did a writeup for TrueNAS on a TerraMaster: https://joelduncan.io/freenas-on-terramaster-f2-221/

Also: https://mightygadget.co.uk/how-to-upgrade-the-terramaster-f4...

It’s an attractive option, because it might be cheaper than a TrueNAS mini from iXSystems (which is also difficult to ship outside US) or a DIY NAS.

You get an affordable TrueNAS server.