Not sure from your question as to whether you simply do not have the cable, in which case a trip to the dealer will sort the problem, or whether your car does not have the AMI system. If it is the latter then yes it can be fitted and Kufatec do a kit https://www.kufatec.co.uk/shop/en/Audi/A7-4G/AMI-Audi-Music-Interface-Retrofit-Audi-A6-4G-/-A7-4G1, note there is a different kit if your car has the 3G+ Sat Nav, and the dealer sorted out getting it installed on my previous C7. May be worth asking a local dealer if they have a "tame" installer as I was told at the time this was a very common retrofit
As per my previous pose AMI can be fitted as it was done on my previous car, the dealer does not know what he is talking about. Where in the country are you? I could put you in touch with the person that fitted mine if it makes sense. Alternatively do a search for retrofit Audi Music Interface and you will find a number of companies offering the service. All of that said you may find that copying your music to an SD Card is just as effective and a lot cheaper
I have not done it (I Have AMI) but this post may help http://www.vwaudiforum.co.uk/forum/showthread.php/168794-itunes-on-SD-card?highlight=music+sd+card, think the critical thing is to ensure the files are MP3 and the iTunes default of AAC
