How to fix the “Back Up Now” button greyed out issue in iTunes on macOS 10.13 High Sierra

Sometimes, for some reason, certain Mac applications will change a setting, turning off the ability to make a backup of your iPhone in iTunes. If your “Back Up Now” button is greyed out, this might be what has happened to you.

To see if this is the case, open Terminal in /Applications/Utilities/ and run:

defaults read DeviceBackupsDisabled

If the response is 1 or TRUE, then this solution applies to you. To turn on the ablity to back up again, just run:

defaults write DeviceBackupsDisabled 0

and the issue will be resolved.