[Scenario]

You have to include the streamer include when you compile the script and load the plugin when you start the SA:MP server.

You can include the streamer include by adding #include <streamer> to the top of your script (below the #include <a_samp> line). Then, you can load the streamer plugin by adding plugins streamer to your server.cfg file, or just add streamer if you already have a "plugins" line.