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 is failing 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 WebUI go to Admin → Projects 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 WebUI before pushing (go to Admin → Projects and browse your project, in the Branches tab you can then create a new branch).
Please note that you need the access right +2 Create Branch in the Push Branch category to create new branches.
Part of Gerrit Error Messages