You can't access that page or toggle that checkbox unless you pay for Copilot first. This might be the most literal case I've ever seen of "if you're not paying, you're the product"
Were you part of the Copilot demo/preview? It appears that doing so leaves your github account in some sort of mostly enrolled state for Copilot that lets you tweak these settings compared to an account that wasn't part of the preview that just takes you to a "start your free trial" page.
