Prettifier cannot check pull requests from forks of private repositories. If your repository is private, forks of it will be private as well and exist outside your organization. Prettifier does not have access to private repositories in other organizations that don’t have it installed. Prettifier needs to know the entire content of files changed in a pull request to format them. This means Prettifier needs read access to the head repository of your pull requests (the repository where the changes were made).