about halfway down are the lines:

canvas .video_game_window -width $video_game_width -height $ video_game_height

The output from your console should look similar to the console output in Figure 13.

That first line should be:

canvas .video_game_window -width $video_game_width -height $video_game_height

The extra space before video_game_height will cause a problem if the student cuts and pastes the code...