Is the same message displayed for every repository you try to fork? If so, then you’ll probably need to contact GitHub support to ask for their help resolving the issue. If not, then you might consider ignoring the problem by selecting another plugin for your contributions.
What experiments have you tried relating to forking repositories on GitHub?
What attributes did you vary in your experiments? Did you try different web browsers? Did you try different computers? Did you try submitting the request from other locations?
What were the results of those experiments?