Nix is a powerful tool to manage packages and create reproducable builds. It also expects /nix on the filesystem to be writable, but starting with macOS Catalina that is no longer the case. But there’s a workaround Create an APFS volume and mount as /nix.
Unfortunately, you might still run into this error:
building '/nix/store/5lrbhhcipg6jjj6rn1kkvn2614jmf5fg-source.drv'...
trying https://github.com/darlinghq/darling/archive/d2cc5fa748003aaa70ad4180fff0a9a85dc65e9b.tar.gz
% Total % Received % Xferd Average Speed Time Time Time Current
Dload Upload Total Spent Left Speed
100 159 100 159 0 0 159 0 0:00:01 --:--:-- 0:00:01 368
100 12.5M 100 12.5M 0 0 1431k 0 0:00:09 0:00:09 --:--:-- 1752k
hash mismatch in fixed-output derivation '/nix/store/f5c24j2dg02zb4386vyrw40hj6v1c051-source':
wanted: sha256:11b51fw47nl505h63bgx5kqiyhf3glhp1q6jkpb6nqfislnzzkrf
got: sha256:0krlrb75fcfa52kx1nvlpyxk060zk2931hfw6g86k5nmzfv9hyxa
cannot build derivation '/nix/store/965ryj5ypzwplvyg8gg37rq63ap53h4s-Libsystem-osx-10.12.6.drv': 1 dependencies couldn't be built
building '/nix/store/96ilwn5cvk7qzi5whmcypr4rqbi448nk-source.drv'...Nix has may long-standing issues with case-sensitivity and macOS.
Remake the APFS “Nix” volume as case-insensitive.