With this error message Gerrit rejects a push if the remote branch can’t be fast forwarded onto the pushed commit. This is the case if the pushed commit is not based on the current tip of the remote branch.
If a non-fast forward update would be done, all commits from the remote branch that succeed the base commit of the pushed commit would be removed. This would be especially confusing for other users that have based their work on such a commit. Because of this Git by default does not allow non-fast forward updates.
When working with Gerrit, this error can only occur if code review is bypassed.
There are different reasons why this error can occur:
the remote branch has evolved since you started your development
you are pushing the commit to the wrong project
You start your development based on the current tip of the remote branch. While you implement your feature / bug-fix, a change in Gerrit gets submitted (or another user directly pushes a commit) so that the remote branch evolves. If you are now pushing your commit, with bypassing code review, your push will be rejected with the error message non-fast forward. To solve the problem you have to either
rebase your commit on the new tip of the remote branch or
merge your commit with the new tip of the remote branch.
Afterwards the push should be successful.
If you do a commit in one project and then accidentally push this commit, with bypassing code review, to another project, this will fail with the error message non-fast forward. To fix the problem you should check the push specification and verify that you are pushing the commit to the correct project.
Although it is considered bad practice, it is possible to allow non-fast forward updates with Git. For this the remote Git repository has to be configured to not deny non-fast forward updates (set the Git configuration parameter receive.denyNonFastForwards to false). Then it is possible to push a non-fast forward update by using the --force option.
Part of Gerrit Error Messages