If your repository is stored in GitHub, Screenshotbot might need access to your GitHub for multiple reasons.
First, if Screenshotbot has access to your repository metadata (mostly just commit messages), Screenshotbot will be able to determine whom to assign issues to based on who wrote the commit.
Second, if you use Github Issues, Screenshotbot can be configured to create issues for any screenshot change.
Install from GitHub marketplace
In order to give access to Screenshotbot, you can install the Screenshot app from the Github marketplace.
To enable GitHub issues, go to Settings | GitHub and check Enable GitHub Issues.