Line 137

old:
if (!isFiring && Vector3.Distance(transform.position, targetPos) <= fireDistance)

new:
if (!isFiring && Vector3.Distance(transform.position, new Vector3(targetPos.x, transform.position.y, targetPos.z)) <= fireDistance)

when you set up your config, it is possible that the rockets will never fire if the fire distance is too low, the height of the plane is too high or the speed is too fast. i suggest calculate the distance between the two points while keeping the height the same.