analogWrite(enablePin, speed); digitalWrite(in1Pin, !reverse); digitalWrite(in2Pin, reverse);
Step on the gas, then put the car in gear. Not good for the transmission.
Typically, you'd have an input pin on both sides connected, to control both motors.
Check both inputs with a multimeter while the program runs - its seems one
isn’t responding, perhaps a loose connection or a short?
Have you got adequate decoupling on the chip?