Just see this on their site
A REPLY OF THE MANCHESTER CITY
On the 300 tickets purchased in the "local" sectors
Following the request made by Atalanta who tried to make Manchester City aware of a solution regarding the 300 tickets purchased by the Atalantini fans in the sectors of the Etihad Stadium reserved for local supporters, the British company announced that they had to cancel the tickets for both legal and security reasons and not being able to allow 300 Nerazzurri fans access to the stadium.
The City added that according to British safety regulations there must be separate areas between local fans and guest fans: as there is no other sector to be reserved for Nerazzurri fans, the British company has therefore canceled the 300 tickets.
Atalanta B.C. he had already asked Manchester City to be able to get at least 4,000 tickets to be allocated to his fans, but the capacity of the Guest Sector reported by the English Company was immediately 2,700 seats without the possibility of increasing the number, nor of allocating further tickets outside the guest sector to Nerazzurri fans.
Following the response from Manchester City, atalantini fans who do not have a valid admission ticket are invited not to show up at the Etihad Stadium gates to avoid unpleasant situations