Lock file error

Since today I’m getting

sudo nixos-rebuild switch                          
building the system configuration...
error: lock file '/nix/store/ncas0cl39dnz05vmcxkgb1nqzrqylnxb-source//flake.lock' has unsupported version 7

How do I fix it?

You need to upgrade to a more recent version of Nix that supports the new lock file format.

nix profile install nix#nixFlakes                    
error: lock file '/nix/store/im96fvb0is9bp8pq42d2mr1w2b9pcb4w-source//flake.lock' has unsupported version 6

That’s the problem:to update to a version that support the more recent lock file I need the flake with the most recent lock format

I guess nixos-rebuild has hard coded a older version of nix than your profile.

You can try to run

nix shell '.#nixosConfigurations.default.config.system.build.toplevel' -c switch-to-configuration swtich

to build and activate your system configuration with your current version of nix.

Of No because I have an older ofnix

Sorry, I thought only the rebuild was the problem. In this case, you can pull in a newer nix version from nixpkgs:

nix shell 'nixpks#nixFlakes' -c nix shell '.#nixosConfigurations.default.config.system.build.toplevel' -c switch-to-configuration swtich

or

nix profile install nixpkgs#nixFlakes
nix shell '.#nixosConfigurations.default.config.system.build.toplevel' -c switch-to-configuration swtich

That too failed because also the nix flake has a version too high

That is strange.

When I used a old build of nixpkgs#nixFlakes I get the same error as you do:

% nix shell 'github:NixOS/nixpkgs/c86af902f443a7f0813467e734b0dcec92e9c2f9#nixFlakes' -c nix build nix
error: lock file '/nix/store/3zpa5parp65vjif5819qz6sqr7grasz5-source//flake.lock' has unsupported version 6

But with a current nixpkgs#nixFlakes build it works as intended:

% nix shell 'github:NixOS/nixpkgs/c86af902f443a7f0813467e734b0dcec92e9c2f9#nixFlakes' -c nix shell 'nixpkgs#nixFlakes' -c nix build nix
% readlink result
/nix/store/0fl25z041j5j7n85jp9a8zw35ryl542n-nix-2.4pre20200701_6ff9aa8

Did you pin the nixpkgs flake to a fixed version via nix registry pin?

What is the output of nix flake info nixpkgs for you?

% nix flake info nixpkgs
Resolved URL:  github:NixOS/nixpkgs
Locked URL:    github:NixOS/nixpkgs/730df00dfb6a22ed0b214ff99370cea250691d17
Description:   A collection of packages for the Nix package manager
Path:          /nix/store/icgx623izb73hsnh6qsd8bn9md27j1vk-source
Revision:      730df00dfb6a22ed0b214ff99370cea250691d17
Last modified: 2020-07-06 14:31:19