I'm tying to back up my iPhone 6 to my Windows 10 PC, but whenever I do, it tells me the backup session failed. This is a relatively new computer, and I have not backed it up to this computer before. Both my phone and my iTunes software are up to date. I need to get this done because I have a battery replacement appointment tomorrow.
iPhone 6, iOS 12.0.1
Actually verify that you have iTunes 12.8.0.150 or later installed.
Just before backing up, reboot your computer and temporarily disable your antivirus (Resolve issues between iTunes and third-party security software - Apple Support)
If it still fails, reinstall iTunes. First delete it using the Windows control panel, then download a new copy from https://apple.com/itunes. Note that this will not delete any iTunes content; it only removes and reinstalls the application itself.
Note that to make a complete backup you need to check Encrypt Backup. If you don't health data, passwords and other personal and private data will not be included in the backup. About encrypted backups in iTunes - Apple Support
When making an encrypted backup don't forget the password! You will not be able to restore the backup without it, and there is no way to recover it.
If you can't get it to work, or just for "belt and suspenders" safety, also make an iCloud backup.
General information: About backups in iCloud and iTunes - Apple Support
Backup session failed
