Thats a browser/caching issue. Just hit your reload button on your browser and it will clear the browser cache. Make sure and visit the link you’re unable to reply to and click your reload button.
Also make sure that you’re not trying to reply on a closed thread. It should say it at the top whether its open or closed. Once a thread/post has been resolved it gets closed to prevent other users from replying on threads that are already resolved. Users are supposed to start new threads instead of replying on others threads, this keeps everything separate and organized, which makes it easier to search the forum for topics – https://appfinite.com/topic/forum-rules/