Why does my gear shift stick in park?

most modern cars have a safty feature that prevents them from being taken out of park with out the foot brake being applied. my best guess would be there is something wrong with this system that stops it from recognising it has been applied. i would have it looked at by a profesional as it can be very tricky to fix if you not familiar with these sorts of things.

I have a 2008 impala and my gear shift would move with out foot on break and locks in park it's due to the screw to th nob