https://github.com/nemccarthy/stash-pullrequest-builder-plugin/issues/150

There has been no activity for almost 2 years, there is a bunch of open issues and PRs that are not anwsered in any way.
The blog post linked from README is down: http://blog.nemccarthy.me/?p=387

Please consider marking the plugin as not actively maintained, then it might get adopted by somebody https://wiki.jenkins.io/display/JENKINS/Adopt+a+Plugin