Did you assign the process variables properly in the designer ? Can you please post a screenshot/
Did you mean assign variable in data association in human task flow? If that what you mean, I already assign the variable.
This is a bit of an old thread, but do you have "Make notifications secure (exclude details)" enabled in the human task?
Open the human task >> Notifications >> uncheck "Make notifications secure (exclude details)" >> redeploy the process. The notifications will contain an HTML form.