On the “Reposettings” page, if the GitHub integration isn’t enabled for the repo, you get an error message saying:
GitHub Integration is installed. However, this repository is not enabled.
However, it should link to: https://github.com/apps/codecov/installations/:installation_id
The former is only available to organization admins, while the latter allows any org user to request that the app be installed for that repo (this flow allows the org admins to approve the request)