This is caused by the steering lock not having been released. The steering lock knob is located on the front of the steering tiller where it meets the scooter deck. The purpose of the steering lock is so that the folded steering tiller can be used a handle when lifting it into a car, etc. Without being locked, the steering tiller would swing back and forth very inconveniently.
To unlock the steering tiller, grasp the knob, pull it out and turn it either direction. The knob will remain in the “out” position. The steering will now be unlocked and the scooter will now be capable of making complete left/right turns.