I'm wondering the same thing! Xcode Cloud seems great, but we need some way of populating the release notes in TestFlight. I have to believe there is a way.

I encountered this issue as well. The fix, in my case: I set up the Workflow, but I was not the owner of the GitHub repo. An email was sent to the owner, requesting them to grant access to the applicable repos set during the Workflow setup. After the owner granted access, I had to right-click the Workflow in Xcode, click Manage Repositories, then in App Store Connect click Grant

Also looking for a solution to this!

I'm also looking for a way to push a tag to origin whenever a build is published to TestFlight. I tried @leolobato's suggestion of using the GitHub PAT, but it was unsuccessful. After watching WWDC '21 Customize Your Advanced Xcode Cloud Workflows, it seems that the source code is not available in the environment where the post-build script is run. Therefore, I'm beginning think the path forward is to use a webhook combined with a microservice that interfaces with the GitHub API.