Code P0963, transmission does not shift into fourth just seems like the engine just revs up?

Hello, I have been having so much trouble with the slipping at 35 or 40 miles per hour, does not go into 4th just seems like the engine revs up, I have flushed it changed solenoids and much more, now I am being told by the auto place I got it from in 2019 that if he remembers correctly they had a problem like that before I bought it and it was the throttle body, could this be the problem, also was told it could be in safe mode or limp mode is this true of so how do I get it out of this mode please help. now I am seeing that the code P0963 has something to do with a heater in the driver chair when I was told it was saying something about the solenoid. please help