There are several reasons:
video is private. Sorry, we can not deal with it.
video is unavailable. We are studying this case. We will fix it if possible.
you paste wrong type of links. At this moment we support downloading from pages which contain a single video. If you paste a user profile link or something else, you will see error.
something is wrong on our side. If you are sure that your link is correct, try downloading one more time.