I have a private repo. I noticed I cannot fork it. I cannot find any information that says I shouldn't be able to. When I go to Options the ability to fix this is disabled. I am guessing you need to pay or something which is fine, but it doesn't say that anywhere...
Cannot click!
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
As of 1/9/2022, I had to update "Member Privileges" in the Organization first:
And then, in the repository it seemed to automatically allow forking. Just note that you might want to update any other repository that you shouldn't be forked.
