Why can’t I reply to the 6th reply to a comment on a discussion thread?
There's no button for a reply at that point. Does it just take up too much space? I mean that as in the reply to the reply to the reply to the reply to the reply to the original comment. Sorry I don’t know how to phrase that better :p
You found the limit to how far Duolingo will allow the replies to nest.
Usually, people just continue the reply but start it from the original comment.
This is part of the reason that having comments listed chronologically rather than by popularity may be helpful.