Sign into iTunes Store keeps popping up
I've recently purchased a new iPad. I synced it with my old one with iCloud and everything seems fine except for one thing. I keep getting a dialogue box popping up asking me to "sign into iTunes Store". It is a very old account and asks me for the password which I have no idea what it is. I'm signed in to iTunes on the account I've been using for years. This box pops up about every two or three minutes. I don't know how to make it go away. Any help would be appreciated. Thanks.
iPad Air Wi-Fi, iOS 10.3.1