Assuming this is for the Verdi festival, the site says the following:
[from this page: http://www.teatroregioparma.org/verdifest/biglietti.htm ]

2011 subscribers have the right to buy subscriptions from September 3 to 13; new subscriptions may be available from Sept 14 to 15; tickets will be available at the box office from September 17 and online from September 18.

So if you are not in Parma soon, you will have to wait until the 18th to buy online.