Matlab doesn't know if taoc is positive or negative. If taoc is negative, the calculation can't really be done. That's because the standard laplace transform assumes f(t) = 0 for negative t, and if taoc is negative, the heaviside function goes into negative t. Assuming w0 is positive, then
syms t vdd s w0
syms taoc positive
v = (1-cos(w0*t))*(heaviside(t)-heaviside(t-taoc));
Z = laplace(v,t,s)
Z = subs(Z,taoc,10*pi/w0)
exp(-(10*pi*s)/w0)*(s/(s^2 + w0^2) - 1/s) - s/(s^2 + w0^2) + 1/s
I left out a couple of constants for simplicity's sake. Since taoc is an exact multiple of pi, the answer is simpler than it would be otherwise.