I've started my window by stating:
Display.setTitle(title);
and then this is where the error occurred:
Display.setDisplayMode(new DisplayMode(width, height));
am I doing something wrong?
You have given us absolutely no relevant information. You have given two method invocations without the values of the parameters and then stated that "the error" occurs. We have no idea what "the error" is and so cannot possibly be of any assistance. Start here for basic use of the Display class: http://lwjgl.org/wiki/index.php?title=LWJGL_Basics_1_(The_Display) (http://lwjgl.org/wiki/index.php?title=LWJGL_Basics_1_(The_Display))