Try these steps to resolve the issue:
Delete the NetServices Keychain item.
- Quit Aperture if it is open.
- In the Finder, choose Go > Utilities.
- Double-click the Keychain Access application in the Utilities folder.
- Click Passwords in the Category list on the left side of the window.
- Click the NetServices item in the main window and press the delete key to remove it.
- Open Aperture and test.
If the above steps don't resolve the issue, turn on Express Checkout.
Turn on Express Checkout
- Quit Aperture.
- Go to the Apple Online Store.
- From the Account menu to the left of the Cart icon, choose "View Account."
- In the Account Settings section, click "Update Express Checkout settings."
- Log in to your account with your Apple ID and password.
- Turn on Express Checkout.
- Open Aperture and try to place your order again.
If Express Checkout is already turned on, reset your Apple ID password.