error: pathspec 'x' did not match any file(s) known to git

error: pathspec 'X' did not match any file(s) known to git means git checkout X found neither a branch, tag or commit called X nor a tracked file with that path. Most often the branch exists only on the remote: run git fetch, then git switch X. If the branch should be new, create it with git switch -c X.

Reproduce it

git init -q -b main repo && cd repo
echo a > a.txt && git add a.txt && git commit -qm init
git checkout feature-login

error: pathspec 'feature-login' did not match any file(s) known to git

Fix: fetch the remote branch

git init -q --bare origin.git
git clone -q origin.git dev 2>/dev/null && cd dev
git switch -q -c main && echo a > a.txt && git add a.txt && git commit -qm init && git push -q origin main
git switch -q -c feature-login && git push -q origin feature-login
cd .. && git clone -q -b main origin.git me && cd me
git branch -r
git fetch -q
git switch feature-login

  origin/feature-login
  origin/main
Switched to a new branch 'feature-login'
branch 'feature-login' set up to track 'origin/feature-login'.

When exactly one remote has a branch with that name, git switch NAME (and git checkout NAME) creates a local branch that tracks it.

The same error for files

git add and git rm report a missing path with slightly different wording and exit status 128:

git init -q -b main repo && cd repo
git add docs/readme.md

fatal: pathspec 'docs/readme.md' did not match any files