Kai, are you trying to post from a phone, tablet, or computer? If you are looking at a non-archived topic, you should be able to scroll to the bottom and you will see a box which says "New Post." It won't be on the teal bar but just above it.
You may also want to confirm you are at the end of the topics. Each page holds about 25 posts so it could be that you are not at the last page if it is a very popular topic such as Cinematography. One way to check is to select the Jump To drop down (which is on the teal bar) and select "Jump to Today." Then try scrolling down and seeing if you see the New Post box.