Only random signal has PSD?
Autocovariance is defined on random signals. 

$$
C_{XX}(t,s) = E\left[(X_t - \mu_t)(X_s - \mu_s)\right] = E[X_t X_s] - \mu_t \mu_s
$$

The expected value only makes sense for random processes. You could treat a deterministic process as a random process where everything except for the one signal you observed has probability zero.