What is the issue???

I have a Code of P0456 on my 2014 Nissan Sentra with 116,400 miles. I
went for 120,000 mile maintenance package (which I thought would fix the
problem, because the mechanic I spoke to on the phone ensured me it
would) at dealership yesterday. Now today, I cannot even drive the car,
because if I stop at a light or stop sign, the car acts as if it will not move
unless you apply ample pressure to gas pedal. Even then the car will barely
accelerate and it seems to hesitate before it will pick up speed and that is a
struggle because it seems like it will try to go but give up then try then give
up, causing a jerking motion. eventually it will catch and accelerate but it will
rev up, hesitate, then accelerate. I have no clue what is going. Could this
code have something to do with how it is currently operating? Could it be
something else? I was told it might be the transmission by the dealership.

*side note the check engine light has been on for a week, and the dealership
would not run diag test unless I paid more money about $260, which I did not
have at the time. However, I purchased my own reader a few months back
and that was the code that I got after check engine light came on.