So this seems basically to be the same problem here Checkout error with working tree clean only none of the solutions work.
Here is my terminal output
username folder % git status
On branch feature/branchname
Your branch is up to date with 'origin/feature/branchname'.
nothing to commit, working tree clean
username folder % git checkout staging
error: Your local changes to the following files would be overwritten by checkout:
apps/content/package.json
Please commit your changes or stash them before you switch branches.
Aborting
username folder % git update-index --no-assume-unchanged apps/content/package.json
username folder % git checkout staging
error: Your local changes to the following files would be overwritten by checkout:
apps/content/package.json
Please commit your changes or stash them before you switch branches.
Aborting
username folder % git checkout -f staging
error: Entry 'apps/content/package.json' not uptodate. Cannot merge.
username folder % git stash
No local changes to save
username folder % git checkout staging
error: Your local changes to the following files would be overwritten by checkout:
apps/content/package.json
Please commit your changes or stash them before you switch branches.
Aborting
So I do status, working tree clean, I do checkout get message file needs to be committed or stashed, I stash, checkout again same message (I have also tried committing without success) and so forth.