Changing forking mode is only available for private repository !
In the Administration tab, I would change forking mode from "Allow only private forks" to "Allow forks". But it was not possible until I check "This is a private repository"
I think that the condition is reversed.
Workaround: check "private repo", change the forking mode and then uncheck "private". The forking mode is saved.
For public repository, the widgets are: see unchecked.png
After checking the "private repo" box: see checked.png