mechanical problem with car I bought within four month

i bought a second hand car in September 2020, Audi A3 1.4 TFSI. the same month the stated to show error message "Vehicle Electronics Malfunction! Shift to P before leaving car". then return the car for them to fix, but that error still come and go. now the car is having another problem loosing power and turn off while driving and smoking in the exhaust. i send them mail they rep**** they cant help me. since i bought the car is only four month all these things mechanical things happen. since i bought the i never open and tempered with engine.