Why do i get "Error using fsurf (line 110) Input must be a function or functions of two variables."

This is the code, but I don't get what the problem is. Why doesn't it make me the plot that I need?

for i = sym(1) : sym(2) : sym(20)
    for j = sym(1) : sym(2) : sym(20)
        sum_w = sum_w + subs(subs(w, m, i), n, j) * 1000;
        wmax_exato = vpa(subs(subs(sum_w, y, b/2), x, a/2));
    end
end

wmax_exato;

m_max = sym(20);
n_max = sym(20);
wmax = sym(zeros(100, 4));

termos = sym(0);
sum_w = sym(0);
for i = sym(1) : sym(2) : sym(5)
    for j = sym(1) : sym(2) : sym(5)
        termos = termos + 1;
        sum_w = sum_w + subs(subs(w, m, i), n, j) * 1000;
        wmax(termos, 1) = i;
        wmax(termos, 2) = j;
        wmax(termos, 3) = vpa(subs(subs(sum_w, y, b/2), x, a/2));
        wmax(termos, 4) = vpa((wmax(termos, 3) - wmax_exato)/wmax_exato * 100);
    end
end

wmax(1:termos, 1:4)

figure
fsurf((symfun(-sum_w, [x, y])), double([sym(0), a]), double([sym(0), b]));