Why does "your purchase cannot be completed" come up on my screen

We are fellow users, we won't know why - if you haven't already done so contact iTunes Support as instructed : http://www.apple.com/support/itunes/ww

Hello Purchase problems,

Thanks for the question. Have you recently moved or changed addresses? Sometimes if the iTunes Store does not have up-to-date information like your new zip code, your payment may be declined:

When there is a mismatch between the iTunes Store's records and your credit card company's records, you will receive an error message stating that your credit card's security code isn't valid or that your zip code does not match your bank's records.

In most cases, this issue is caused by a formatting error in the billing address that you have provided. You can correct the issue by verifying that the information you have provided is correct, or by changing the format of your address to match the format that your credit card company has on file.

via iTunes Store: My credit card's security code or zip code does not match my bank's records

http://support.apple.com/kb/TS1646

As with many other merchants, the iTunes Store and Mac App Store places an authorization hold on your payment card account for the approximate amount of any purchase you make. If you update your billing information, the iTunes Store or Mac App Store will place an authorization hold on your payment card account, usually in the amount of 1 USD or the local currency equivalent, to verify your account information.

via iTunes Store & Mac App Store: About payment card authorization holds

http://support.apple.com/kb/HT3702

You can also learn about choosing “None” as a payment option:

Why can’t I select None when I edit my payment information?

http://support.apple.com/kb/TS5366

Thanks,

Matt M.

Why your purchase cannot be completed showing on my screen