You forgot to use the variables you just created:
x = 0:100;
y = linspace(50,150,101);
[X,Y] = meshgrid(x,y);
f = cos((pi.*X)./50).*sin((pi.*Y)./100);
% ^ not x, but X ^ not y, but Y
surf(X,Y,f)
You should also avoid the habit of starting your code with clear all,close all,clc. Especially that first one should be avoided. During debugging you can use clear or clearvars to remove variables, otherwise use functions.
