What is iTunes error code -12842 ?
What can I do about it?
iPhone 6s Plus, iOS 12
If you see an error in iTunes on your Mac or PC - "When you use iTunes on your Mac or PC, you might see an error code or alert message. You can fix most errors with these steps." - Very general tips on updating software
https://support.apple.com/en-us/HT205724
Find an iTunes error code
https://support.apple.com/en-us/HT203174
but I don't think that one is in there. I have about 30 codes listed for iTunes errors and that isn't among them. Are you sure you quoted it correctly? What are you doing when you see this?