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

If it was your intention to create a new branch you can either

Please note that you need to be granted the 'Create reference' access to create new branches.

Part of Gerrit Error Messages