"Unknown error"

Settings>iTunes & App Store>Apple ID. Tap your ID and sign out. Restart the iPad by holding down on the sleep button until the red slider appears and then slide to shut off. To power up hold the sleep button until the Apple logo appears and let go of the button.

Go back to Settings>iTunes & App Store>Sign in and then try again.

I received this email from Apple and this corrected the problem for me... I only had to reset the iTunes cache

1) Open iTunes

2) At the top menu, click File > Preferences (iTunes > Preferences on a Mac)

3) In the Preferences menu, click the Advanced tab

4) In the Advanced menu, you should see a button that says "Reset Cache". Please click this button.