This has tricked many people already, and today me: The Github user given in the "account" config field needs to have "Write" permissions to the repository; having Read permissions plus explicit "repo:status" permissions is not enough.
"Users with push access can create commit statuses for a given ref:"
This caused me a couple hours of confusion today, and also confused other projects, e.g. https://github.com/jonnyzzz/TeamCity.GitHub/issues/12
It would be great if the README that gets rendered onto https://wiki.jenkins-ci.org/display/JENKINS/Pipeline+Githubnotify+Step+Plugin could say explicitly that the user given with "account" has to have Write permission to the repo.