The error in your code arises from the fact that the variable nu is a symbolic variable defined using syms, while x is a numeric array (kr). The besselj function requires both inputs to have the same size or one to be a scalar.
To fix this, you can use a for loop to evaluate the expression for each value of i and radius like this:
c = 3e8;
f = 6e9;
n = 10;
i = 1:n;
Wl = c / f;
k = (2 * pi) / Wl;
radius = 1e-2:1e-4:6e-2;
r = length(radius);
kr = k * radius;
X = radius / Wl;
Ae = zeros(1, r);
for i = 1:n
B = zeros(1, r);
for j = 1:r
x = kr(j);
H = besselj(i, x) - 1j * bessely(i, x);
dH = (1/2) * (besselj(i-1, x) - besselj(i+1, x) - 1j * (bessely(i-1, x) - bessely(i+1, x)));
ht = H * dH
B(j) = subs(ht, x, kr(j))
end
A = (abs(((-1)^i) * (2 * i + 1) ./ B).^2) / (4 * pi);
Ae = A + Ae;
end
figure(1);
plot(X, Ae);
