Why is Reply greyed out on my thread?

I was asked to provide information regarding my NOBU problem via a private message. That person/private message is no longer available & Norton advised I post further information to my thread, but Reply at top left of thread is greyed out. How can I post any further info to the thread? 

But also -  why would Norton ask me to post private information that previously required a private message, to solve the problem on a public thread???