Nixos-rebuild switch --show-trace - what is the error? where is it listed

nixos-rebuild switch --show-trace

building Nix...
building the system configuration...
updating GRUB 2 menu...
cannot copy /nix/store/9pwq0j9q4frd0808xvw56r8xj20bnxps-initrd-linux-5.6.18/initrd to /boot/kernels/9pwq0j9q4frd0808xvw56r8xj20bnxps-
warning: error(s) occurred while switching to the new configuration

looks like it is locked?

ok, there is an error but what is the error?

That is the error. Is your /boot partition out of space or something like that?

How to let nixos clean up the boot partition?

do you run periodically sudo nix-collect-garbage -d or in other ways remove old nixos generations? If you do, then nixos-rebuild will clenaup EFI boot partition automatically.

It might be that this time you should clean a bit of it manually, but usually it should work automatically.

Alternatively, enlarge boot filesystem.

not any more if it is once filled up
well so I had to manually clean it up.