Neither of those.
If your API is well-designed, the URL includes the name of the city, e.g.
since IP geolocation is unreliable, your users might be using VPNs, your users might want hail a ride for someone else, etc. Suggesting a city based on the user's location is the API client's responsibility. Usually, the client has much better resources for determining the user's location anyway (for example, a mobile device's location service).
Once you've done that, the correct answer to
becomes obvious: 404 Not Found: No resource for hailing a ride at SomeUnsupportedCity exists.