If you leave the leads disconnected and only toggle the GPIO pins, do the LED’s turn on for each output you want to use?
They will operate without the VIN and GND connection. They are powered from the GPIO outputs.
Try this next, without the motors being connected. Connect VIN to BATT+ and GND to BATT-
With the GPIO output off (low), measure with a multimeter on output 1 and it should be close to the battery voltage (less the diode drop)
Now switch the GPIO for output 1 to on (high) and the output 1 should now be close to zero volts. If this works, your outputs are switching.