I am trying to push sandbox threads for the libqmi and modemmanager-next repositories in third_party, but I get the following issue when I try to create them:

(cr) (qrtr-control-socket) ejcaruso@ejcaruso ~/trunk/src/third_party/modemmanager-next $ git push cros HEAD:refs/sandbox/ejcaruso/qrtr-socket-181115
Enumerating objects: 7, done.
Counting objects: 100% (7/7), done.
Delta compression using up to 24 threads.
Compressing objects: 100% (4/4), done.
Writing objects: 100% (4/4), 605 bytes | 605.00 KiB/s, done.
Total 4 (delta 3), reused 0 (delta 0)
remote: Resolving deltas: 100% (3/3)
remote: Processing changes: refs: 1, done
To https://chromium.googlesource.com/chromiumos/third_party/modemmanager-next
 ! [remote rejected]   HEAD -> refs/sandbox/ejcaruso/qrtr-socket-181115 (prohibited by Gerrit: not permitted: create)
error: failed to push some refs to 'https://chromium.googlesource.com/chromiumos/third_party/modemmanager-next'

I should be able to create sandbox branches no matter what the repo is, I thought.