$u(t)=t, G_0(s)=\frac{k_p}{s}, G_R(s)=k_f$
Find parameter $k_f$ for which steady-state error equals 0
Finding $E(s)$
$$E(s) = U(s)*\frac{1}{1+\frac{k_p}{s}*k_f} = \frac{1}{s^2+k_pk_f*s}$$
Steady-state error:
$$\lim\limits_{s \to 0} s*\frac{1}{s^2+k_pk_f*s}=\lim\limits_{s \to 0} \frac{1}{s+k_pk_f}=\frac{1}{k_pk_f}$$
So there's no $k_f$ that exist that would fulfill the requirements, but apparently that's wrong somehow.
Could anyone tell me where I have made a mistake?