An organization owner must allow forks of private and internal repositories on the organization level before you can allow or disallow forks for a specific repository. For more information, see "Managing the forking policy for your organization."
On your GitHub Enterprise Server instance, navigate to the main page of the repository.
Under your repository name, click Settings. If you cannot see the "Settings" tab, select the dropdown menu, then click Settings.
Under "Features", select Allow forking.