Nix cannot find file

While trying to use Nix flakes, you might get an error that looks like this: ¹

warning: Git tree '/home/exodrifter/notes/logbook' is dirty
error (ignored): error: end of string reached
error:
       … while calling the 'derivationStrict' builtin
         at <nix/derivation-internal.nix>:9:12:
            8|
            9|   strict = derivationStrict drvAttrs;
             |            ^
           10|

       … while evaluating derivation 'quartz-4.3.1'
         whose name attribute is located at /nix/store/wzx1ba5hqqfa23vfrvqmfmkpj25p37mr-source/pkgs/stdenv/generic/make-derivation.nix:331:7

       … while evaluating attribute 'installPhase' of derivation 'quartz-4.3.1'
         at /nix/store/bkm5f4jk8c8w7zg4iav1vv2xrxvisv9r-source/flake.nix:36:15:
           35|
           36|               installPhase = ''
             |               ^
           37|                 runHook preInstall

       error: path '/nix/store/bkm5f4jk8c8w7zg4iav1vv2xrxvisv9r-source/quartz.layout.ts' does not exist

Nix errors contain the most relevant error at the bottom; here, we can see that it cannot find the file quartz.layout.ts. In this example, the file quartz.layout.ts does exist, but files that are not tracked by the git repository are not copied to the Nix store by the Nix flake. Tracking the file (such as git add quartz.layout.ts for example), then the Nix error will go away. ²