AWS Amplify only lists repositories that you've explicitly granted it access to. If the repo is under a GitHub organization, you'll need to make sure Amplify has permission to access organization-level repositories. This is usually handled during the GitHub authorization step when connecting your account to Amplify.

Try disconnecting and reconnecting GitHub within Amplify, and when prompted, ensure you grant access to the specific organization where the repo is hosted. Also, double-check your role in the organization—depending on the settings, being a collaborator might not be enough to expose the repo through Amplify.

You can refer to AWS’s official documentation on this here:
https://docs.aws.amazon.com/amplify/latest/userguide/hosting-continuous-deploy.html#step-2-connect-repository