The Code was right for the most part except that you did not make the time step small enough and the bug could be easily fixed by changing the timestep to .007 or smaller if you want but .007 is sufficient enough.
F0 =13;
T0 =1/F0;
Ts = 0.007;
t = 0:Ts:13*T0;
x = real(exp(j*(2*pi*F0*t-pi/2)));
plot(t,x)
