you are not allowed to upload merges

With this error message Gerrit rejects to push a merge commit if the pushing user has no permissions to upload merge commits for the project to which the push is done.

If you need to upload merge commits, you can contact one of the project owners and request for this project permissions to upload merge commits (access right +3 Upload merges permission in the Read Access category).

If one of your changes could not be merged in Gerrit due to conflicts and you created the merge commit to resolve the conflicts, you might want to revert the merge and instead of this do a rebase.