On your model Impala, there is a shift interlock override.
1. You need to pop the shifter surrounding trim. Insert a small flat screwdriver between the trim and console base then pop it up, working your way around.
2. Unscrew the shifter knob from the shifter. There is one philips screw in the front of the knob. Remove the knob then remove the trim. Note the shifter lock shaft within the lever.
3. Under the shifter trim is a chrome-colored button, the interlock solenoid, which faces the passenger side. Hold that button to override the interlock, use another hand to push down on the exposed shift lock shaft to shift out of park. Its only needed for the park gear.
The usual problem on this car is wires that go to that interlock solenoid are too short and they pull out or break as the shift lever moves. The solenoid is usually still operational. To permanently fix it, you must re-wire the connector and add or pull more slack for the wires to move freely with the shift lever.