So I recently bought a new head unit to my skoda fabia 2016 ambiente, but I just can’t get sound on my rear speakers…. Does someone knows what could be the problem? 

Does the car have rear speakers? Is the unit capable of outputting sound to the rear speakers if they are present?

Yes I have rear speakers and they worked really good with the last head unit and also the unit is capable of outputting the sound

Try adjusting the fade controls in the settings of the unit.

As above or recheck the wiring and plugs as it sounds like one may be loose on the back of the radio or any adapter harness you've fitted