PhuocNguyen-6888, Thanks for good question.
As I understand you’re setting up CI/CD (GitHub) on an existing Azure WebApp for the first time. Just to confirm (isolate the issue), is this a private repo? Is this issue confined only to this repo and WebApp?
It is necessary to do an explicit action to enable a GitHub Organization to be able to be visible from an OAuth connected client.
To begin with, you may browse to https://github.com/settings/applications and review the access permissions as mentioned in this doc - Investigating continuous deployment
And also, from the GitHub settings ‘You need to mark all the organizations that you need access to’ as outlined in this similar discussion thread and see if it helps.