With this error message Gerrit rejects to push a commit for code review if the specified target branch does not exist.
To push a change for code review the commit has to be pushed to the project’s magical
refs/for/'branch' ref (for details have a look at Create Changes). If you specify a non-existing branch in the
refs/for/'branch' ref the push fails with the error message branch … not found.
To fix this problem verify
that the branch name in the push specification is typed correctly (case sensitive) and
that the branch really exists for this project (in the Gerrit Web UI go to Projects > List and browse your project, then click on Branches to see all existing branches).
If it was your intention to create a new branch you can either
bypass code review on push as explained here or
create the new branch in the Gerrit Web UI before pushing (go to Projects > List and browse your project, in the Branches tab you can then create a new branch).
Please note that you need to be granted the Create reference access to create new branches.
Part of Gerrit Error Messages