[Mark McLeod]

You have both motors wired to the x axis of a single joystick. The first output of Index Array unless overridden by an explicit index is Axis 0 or the x axis on a flight stick.
You probably want to use the normal y axis instead.
Either wire from the second output (y-axis), rather than the first output on the Index Array, or add a constant on the left side of both Index Arrays of "1"

Since you are only using one joystick, but using it for tank drive, your robot will only ever go forwards and backwards.
You don't have anything to turn.
It's more common to use two joysticks or at least two different sticks on an xBox-like controller to control the two sides separately.

If you want arcade drive, then you'll have to mix the x/y of the single joystick yourself, since you're avoiding 2 Motor Drive/Arcade.