what happens when you connect the usb cord from the ipad to the computer
first try :Log out of Apple ID account. Goto Settings > iTunes and Apps Stores. Tap your Apple ID and select Signout. Shutdown. Wait 10 seconds. Restart. Login to Apple ID. Now, try downloading again.
sometimes the date and the time can be correct verify that..if its incorrect you can correct it by
settings>general>date & time>set date & time
please let me know if this was helpful