Untrack files from git in worktree

Untrack files from git in worktree

I have three machine, local -> Server A -> Server B
and programming on local, then use git push local code to Server A,
on Server A I set a hook to push the code to Server B,
both on Server A and Server B have configure this
$ git config core.worktree /home/www
$ git config core.bare false
$ git config receive.denycurrentbranch ignore
and have a hook git checkout -f
how can I change some files on Server B and not track it?
I try git update-index --assume-unchanged path/to/file
but alway notice fatal: Unable to write new index file
I'm sure user have permission in that folder(can write, read and execute)
I also try git update-index --skip-worktree path/to/file doesn't work