The 27-year-old actress was said to be booked for the event to promote "The Canyons" and no reason was given for her absence.
The actress, who completed a 90-day stay in court-ordered rehab last month, insisted she wish she could have been part of the festival.
"I was never confirmed to attend. Of course, I would have enjoyed returning to that wonderful city and being a part of this amazing event, but my focus is on my health and well-being. Plain and simple, it is of the utmost importance," she wrote on her website.