Is it possible to write that
I wish I could have seen you when you are playing with your band in Italy next week but it will not be possible.(think this one is not correct) could have seen refers to the past situation
I wish I could see you when you are playing with your band in Italy next week but it will not be possible.(think this the best could see refers or can refer to a future situation
I wish I saw you when you are playing with your band in Italy next week but it will not be possible (think the last one is incorrect because "saw"refers to the present situation