Your schematic shows arduino d6 & d7 connected to IN3 & IN4.
Your photo shows the a sensor shield connected . I don't see any processor chips on that sensor shield so it must be controlled by the arduino. Why is the motor driver connected to the sensor shield ?
You said you haven't actually done any testing yet so how did you come up with this wiring ?
Why are you not using analog pins on the arduino for the PWM Enable signals for the motor ? (only one , right?)
What kind of signal is that sensor shield providing to the motor driver ?
Where did you come up with that wiring ?