06 Chevy impala

I would suggest you take it to someplace like Aut zone. See if the vehicle is throwing any transmission codes. They will scan it for free. I am guessing you already checked the fluid levels and verified all fuses are good in the engine compartment. If not, definitely check those two things.

It may just be a bad shift solenoid or bad clutch pack. I'd look for a dedicated transmission shop to take it to and get a professional diagnosis. Stay far away from the chain shops, they'll charge you a fortune just to look at the car. Most small mechanic shops aren't always the best place for transmission work either. A tranny shop can run diagnostics on it and tell you if it's the valve body/solenoids, clutch packs, TC, etc. If they tell you it needs to be rebuilt and that'll be $2,500 thank you very much then go get a second opinion someplace else.