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."
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On your GitHub Enterprise Server instance, navigate to the main page of the repository.
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Under your repository name, click Settings. If you cannot see the "Settings" tab, select the dropdown menu, then click Settings.
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Under "Features", select Allow forking.