Why I am unable to reply to other's post in the Apple Support Community?

If you see this banner at the top of a thread, it indicates a thread is closed and locked due to inactivity:

Spend time on the most recent posts. Most people post once and then go away, meaning that , even if you could answer months or years later, they would not see it.

This would usually indicate that you are attempting to reply to thread that has been closed due to inactivity. Do you have a link to the thread in question?