Hi Santiago,
You are getting this warning because you are attempting to plot a complex input with plot function. If you want to plot imaginary part, I would suggest you pass the imaginary part to the plot function. For reference you may use this code :
T = 1;
W = (2*pi)/T;
t = [-2*pi:0.0001:2*pi];
x_t = 0;
for n = -100:100
C_n = (1/(pi*n))*sin((pi*n)/2)+(1/(1i*2*pi*n))*(exp(-1i*3*pi*n/2)-exp(-1i*pi*n/2));
x_t = x_t + C_n*exp(1i*n*W*t);
end
plot(t,imag(x_t))