Doing monitor.clear returns a function as a value instead of executing it. So computercraft tells you that it is expected of you to assighn the value to something (e.g. local cls = monitor.clear), but you are trying to call the function, so instead you should do monitor.clear() telling it to call the function with no arguments. Functions are also values so in the example I gave earlier cls would become also a function, and you could do cls().