FOURIER is a function that is part of the symbolic toolbox. If you do not have that toolbox, then you could not use it. However, your problem is that you are trying to use it on a sequence of numbers, not on a true square wave. And the function SQUARE is part of the signal processing toolbox. It works with numbers and series of them.
So if you want to compute a symbolic Fourier series, then you need to create symbolic objects. If you want to compute a fast fourier transform, which can be used on pure numbers, then use FFT. But only you know what it is you really wanted to do here.
