How to Fix iTunes Error -45076
Here's how to fix error -45076 if you have downgraded iTunes 12.7 to 12.6.2 still under Sierra and have then upgraded your OS to High Sierra: 


Re-download the iTunes 12.6.2-DMG directly from the Apple Servers.
Make a backup of ~/Music/iTunes/iTunes Library.itl (just in case).
Delete iTunes 12.6.2 from your High Sierra Mac, this time entirely (list of ingredients courtesy of macrumours.com member driveparty):sudo rm -rf
/Library/Documentation/Applications/iTunes/Acknowledgements.rtf
/Library/Documentation/iPod/Acknowledgements.rtf
/Library/Frameworks/iTunesLibrary.framework/
/Applications/iTunes.app/
/System/Library/PrivateFrameworks/iTunesAccess.framework/
/System/Library/LaunchDaemons/com.apple.fpsd.plist
/System/Library/PrivateFrameworks/CoreFP.framework/
/System/Library/PrivateFrameworks/CoreADI.framework/
/System/Library/LaunchDaemons/com.apple.adid.plist
/System/Library/CoreServices/UAUPlugins/ADIUserAccountUpdater.bundle/
/System/Library/CoreServices/CoreTypes.bundle/Contents/Library/MobileDevices.bundle/
/System/Library/LaunchDaemons/com.apple.usbmuxd.plist
/System/Library/PrivateFrameworks/AirTrafficHost.framework/
/System/Library/PrivateFrameworks/DeviceLink.framework/
/System/Library/PrivateFrameworks/MobileDevice.framework/
/System/Library/Extensions/AppleMobileDevice.kext/
/System/Library/Extensions/AppleUsbEthernetHost
Install iTunes 12.6.2 again from the DMG downloaded above.