The setup documentation only specifies installing with ext4 filesystem. I tried following the Nix wiki page on BTRFS, but this command doesn’t work for me: mkfs.btrfs -L nixos -f --sectorsize 16384 /dev/nvme0n1p5 but ext4 command in the documentation, mkfs.ext4 -L nixos /dev/nvme0n1p5 does. I have next to no familiarity with btrfs, I want to use it for snapshots and transparent compression.
Could you elaborate a bit more? What’s the error and how can you reproduce it, which steps?
Just looking at the repo there are several happy people running btrfs, but there could be of course a bug that is hiding in plain sight. And if it’s not a bug it is an opportunity to improve documentation 
I tried it again and it worked.