Because this operation isn't defined on elliptic curves, most specially in a curve defined over a discreet field, this leads to the "discreet logarithm problem" that are an unsolved problem so far.
if you are using a curve over, let's say, the real numbers set, you can calculate it. But for cripto, usually the curves are defined over a set mod(p).

secp256k1 is defined over the set mod(p), where p is: