How can I cansel the purchase?

You can try contacting iTunes Support. Either use the form or scroll to the bottom of the page in the 2nd link. Follow through the screens to get a chat or telephone option.  You may need to pick a problem that is slightly different than yours to get a chat option.  

iTunes Support

iTunes Support – world wide

Hello,

iTunes or App Store purchase: Request a refund for an App Store or iTunes Store purchase

Apple Online Store: Viewing & Changing Orders