From the UEFA regulations:
(ii) If restrictions imposed by the national/local authorities of the home club’s country apply to the visiting club, the home club must propose a suitable alternative venue which may be in a neutral country (within the territory of a UEFA member association) that would allow the match to take place without any restrictions for both clubs. If the home club fails to propose such suitable alternative venue, the club in question will be held responsible for the match not taking place and the match will be declared by the UEFA Control, Ethics and Disciplinary Body to be forfeited by the home club, which will be considered to have lost it 3-0.