Failed to install bootloader warning: error(s) occurred while switching to the new configuration

[waffle8946]

(NixOS) (FIXED) Can't nixos-rebuild switch because /boot is full even after nix-collect-garbage -d

Delete the old kernels manually from /boot (I’d keep at least one old one). Then delete those previous generations (I’d keep 1-2). Then rebuild. Yeah it’s annoying. I keep my /boot at least 500 MiB for this reason.

Yes it’s not ideal but you have to literally manually delete files in /boot, and not even with nix, as the first step.