Central economic planning does not automatically mean that it will always miss consumer needs. It was a big issue in USSR which some academics were planning to address with advancement in cybernetics (which meant something like what we call "big data" today), but it never received proper attention from the government.

Regarding advertising, the customers of advertisement agencies (there were few in USSR) were firms themselves, not Gosplan. Every Soviet firm or Glavk ("main committee" - sort of a business unit in Soviet government responsible for the management of whole market segment) had a plan, that's true, but it was up to them how to implement it, so some did invest in advertisement (examples were beverages, perfume etc etc).