It gives a blank plot because the linspace calls are only producing one element. The third argument to linspace is the number of elements desired in the vector it produces. An argument of 1 returns the beginning point (first argument) and stops.
I am not certain what the desired result is, however here are two possibilities —
n=1; a=1;
x = linspace(0,0.01,25);
y = linspace(0,0.01,25);
z = linspace(0,0.01,25);
[X,Y,Z] = meshgrid(x,y,z);
f = @(x,y,z) 8.*sqrt(sin(n.*pi.*x/a).*sin(n.*pi.*y/a).*sin(n.*pi.*z/a));
figure
scatter3(X(:),Y(:),f(X(:),Y(:),Z(:)), '.')
figure
isosurface(X,Y,Z,f(X,Y,Z), 5E-13)
grid on
view(60,30)
I leave the rest to you.
.
