Hi. Unfortunately, it's not an error. It just means you haven't built a RAM preview or rendered to a codec that your computer can play back. Use Adobe Premiere (or similar) for realtime playback and editing of digital video. Use After Effects for creating simple or complex visual effects or animation -- and expect to tick through the timeline until you load the frames into RAM, or render a movie to disc. Check out some basic tutorials on using After Effects http://www.videocopilot.net/ or read the manual to get started. I believe it's in the help menu.
AE requires to create RAM previews for realtime playback. You really need to read the help or watch some basic tutorials...
Both the above are correct If you're after a quick way of watching videos in real time and you don't have the time yet to watch more tutorials, then for now you can just render out a preview of your video to watch it.
Pressing zero on the numpad will render your timeline using RAM preview. For now this should mean your video is watchable and you can check out tutorials when you get the chance!