Allowing or disallowing users with read access to create discussions
By default, organization members with read access can create discussions if a repository administrator or organization owner enables discussions for a repository owned by the organization.
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In the upper-right corner of GitHub Enterprise Server, select your profile photo, then click Your organizations.  
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Next to the organization, click Settings. 
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In the "Access" section of the sidebar, click Member privileges. 
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In the "Admin repository permissions" section, under "Repository discussions", select or deselect Allow users with read access to create discussions. 
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Click Save.