As it is written in Sonar Qube forum
Pull Request support requires at least the developer edition of SonarQube. If you don’t want to analyze your pull requests and keep using the community edition, then simply create a second build definition without any SonarQube task, and use it in your branch strategy to build pull requests.
And also here
Pull Request analysis is available as part of Developer Edition and higher.
So you can’t do this with community edition.
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