Installation issues on macOS Catalina

I’m trying to install nix on my macbook running macOS 10.15.7 (Catalina). The initial install told me

❯ curl -L | sh
Note: a multi-user installation is possible. See

Installing on macOS >=10.15 requires relocating the store to an apfs volume.
Use sh <(curl -L --darwin-use-unencrypted-nix-store-volume or run the preparation steps manually.

So then I tried

❯ sh <(curl -L --darwin-use-unencrypted-nix-store-volume
Note: a multi-user installation is possible. See
Creating volume and mountpoint /nix.

    | This installer will create a volume for the nix store and        |
    | configure it to mount at /nix.  Follow these steps to uninstall. |

  1. Remove the entry from fstab using 'sudo vifs'
  2. Destroy the data volume using 'diskutil apfs deleteVolume'
  3. Remove the 'nix' line from /etc/synthetic.conf or the file

Configuring /etc/synthetic.conf...
Creating mountpoint for /nix...
Creating a Nix Store volume...
error: refusing to create Nix store volume because the boot volume is
       FileVault encrypted, but encryption-at-rest is not available.
       Manually create a volume for the store and re-run this script.

I checked out the documentation (Introduction), but it doesn’t say anything about this error message or how to work around it.

So how do I proceed doing this manually? And why doesn’t the installer just do it for me automatically?