There are more problems that that one line. When I ran your code, I discovered that most of your (unfortumately-named) ‘sum’ matrix were NaN, so there was no plot. (Please do not give your variables names tthat conflict with MATLAB functions. Naming variables that conflict with MATLAB functions is termed ‘overshadowing’ and makes the function completely useless if you want to use it later. This is what your ‘sum’ variable would do.)
The loops are also unnecessary. You can take advantage of vectorization. Try this version of your code:
v0=100;
a=10;
b=5;
k=1:70;
n=(2*k)-1;
x=1:10/100:10;
y=1:5/100:5;
[X,Y,N]=meshgrid(x,y,k);
Ve = (sin(N.*pi.*X/a).*sinh(N.*pi.*Y/a)) ./ (N.*sinh(N.*pi.*Y/a));
V = (4*v0/pi) * sum(Ve,3);
It works, however it does not likely produce the sort of contour you want, so I leave that to you to sort out.
