According to the British media outlet the Mirror, Arsenal cancelled their pre-match press conference ahead of the Champions League match against Napoli because widespread flight delays were caused by air traffic controls in London, UK.

The Mirror said that Arsenal encountered serious travel disruption while travelling from London to Naples. Like hundreds of other passengers, the entire Arsenal group was affected by flight delays at Gatwick Airport and Heathrow Airport. It is understood that the air traffic control systems near the two London airports experienced a technical failure.
Due to the flight delays, Arsenal will be unable to arrive in Naples in time and therefore cannot attend the pre-match press conference as originally scheduled. However, UEFA regulations also state that plans can be adjusted if uncontrollable circumstances force a team to change its pre-match arrangements.
UEFA's official rules state: "Clubs may only change the arrangements for the day before the match (MD-1) if they are affected by force majeure factors and have received UEFA's approval."
Arsenal will face Napoli away from home at the Diego Armando Maradona Stadium at 3:00 a.m. Beijing time on September 10.