Likely cause is a defective shift solenoid. But it could be due to just a dying battery. Check battery and make sure it's good; you might even put it on a charger to make sure. After this, if the problem persists, take it to a transmission specialist.
Search Chevrolet Impala Questions
Chevrolet Impala Experts
Find great deals from top-rated dealersTMSearch
Related Models For Sale
Used Cars for Sale