To import a book from, or link a book to a GitHub repository belonging to a GitHub organization, the GitHub organization needs to grant access to the GitBook application. Otherwise, the repository will not be listed for you to import.
You can check that GitBook.com is able to access the required GitHub organization by visiting GitBook's authorization page.
If the GitBook application doesn't have access to the GitHub organization, you can either:
After the access has been granted on GitHub, you must update your GitHub Access Token for GitBook.com.
To do so, simply go to your settings page, click on
Reconnect GitHub Account
with access to your public (and private if needed) repositories.