Check spelling... :)
x1=[100 200 300 400]';
[x,y]=meshgrid(x,y1);
You defined x1 but used x as argument instead...
fitsurface=fit([x,y],z, 'poly21')
fit wants vectors, not arrays. Use the Matlab idiom (:) to return all elements of matrix/array as column vector:
x=x(:); y=y(:); z=z(:);
fitsurface=fit([x,y],z, 'poly21')
Somehow the builtin plot doesn't recognize it's a surface and only plots on 2D axis; I had to use scatter3 to show...
Also, it appears the real curvature is in Y but you fit the quadratic in X; looks like should either reverse role of X,Y or use 'poly12' instead of 'poly21'
