I think you are talking about your organisation’s private repo, right? As I didn’t find this checkbox for personal private repositories.
If so, then you need to:
-
go to your organisation’s settings, Member privileges, and check “Allow forking of private repositories”
. -
go to your repo and you’ll see “Allow forking” will be automatically checked after doing step 1,
.
The process is also described here:
https://docs.github.com/en/organizations/managing-organization-settings/managing-the-forking-policy-for-your-organization