I had this problem on my ford focus.  When you press on your gear shift and press the button there is an actuator on your gear shift that releases your gear shifter in order to put it in drive or reverse.  The actuator is released when you press your brake and push the button on your gear shifter to release it.  I took the middle console off and checked the actuator and it was not actuating releasing the gear shift.  The actuator mechanism went bad.  I had to order the hole assembly on line with the gear shifter it only costed $99.  Order it and got it within 3 days, replaced it put it back together and it worked.   The only thing is that you have to take the console off and release the bar that goes to the transmission.  Really easy fix.   If I had taken it to a dealership they would have charged me $2000 as a fix to the transmission not knowing that it was just the shifter actuator that was not working.   Hope this helps.