function [X] = FourierSinc(a,b,K)
w = linspace(-10,10,600);
You assign a list of definite numeric values to w
syms w
You immediately throw away those values and do the equivalent of
w = sym('w')
U = piecewise(abs(w)<a*pi,K, abs(w)>a*pi, 0);
X=exp(-1i*b*w)*U;
If you ever switch back to numeric w, be careful because the left and right side of the * would be row vectors, which would be a problem for the * operator. With your current setting of syms w this is not a problem, but you could make it easier to understand by using .* instead of *
figure, fplot(w,X);
fplot() accepts a symbolic expression or symbolic function (or array of those) in the first parameter; the second parameter is the bounds for plotting.
I would suggest that you want something like
function [X] = FourierSinc(a,b,K)
W = linspace(-10,10,600);
syms w
U = piecewise(abs(w)<a*pi,K, abs(w)>a*pi, 0);
X=exp(-1i*b*w).*U;
figure;
xw = double(subs(X, w, W));
plot(W, xw);
