Can an aeroplane ever be so far away from an airport that it cannot land?
Can a commercial airplane fly outside the safety area? Yes, there is nothing physically stopping the plane from doing so. 

Is it allowed to do so? No, precisely for the event should an engine fail then it will have enough range to get to the airport and make a safe landing.

Extending the allowable range of twin engine aircraft is what ETOPS (Extended-range Twin-engine Operational Performance Standards) is about.

By default commercial airplanes with 2 engines must remain within 60 minutes flying time of an airport in case of engine failure. This was decided at a time where engines were unreliable.

However as engine reliability increased airplanes could have that range increased with an ETOPS certificate.