A parenthesis is missing in your formula (look your print), there is an closing parenthesis after "t", but there is no opening parenthesis associated.
I've tried this, that seems to work:
a=3;
l=0.15;
m=0.5;
D=0.4;
wn=sqrt(9.81/l);
t = 0:0.05:(a*pi/wn);
x0=(D/2)*(1-cos((wn/a)*t));
plot(t,x0);
