Q: why i can't download new GAGA's pre-ordered ARTPOP?

Hello, iMaxLike. 

I experienced this issue with a Pre-order album as well.  I would recommend canceling the Pre-order and download it again as it most likely has not been charged.  This article will walk you though the process.

iTunes Store: About Pre-orders

http://support.apple.com/kb/HT5714

Check your purchase history and you method of payment to make sure that the item has not been deducted prior to processing these steps.

iTunes Store & Mac App Store: Seeing your purchase history and order numbers

http://support.apple.com/kb/HT2727

If the album has been charged, follow the steps in this article to report a problem with this purchase.

How to report an issue with your iTunes Store, App Store, Mac App Store, or iBooks Store purchase

http://support.apple.com/kb/ht1933

Cheers,

Jason H.