back up now greyed out

he options are

1. iTunes --> Remove and reinstall iTunes and related software components for Windows 7 and later
2. Use another cable --> Identify counterfeit or uncertified Lightning connector accessories
3. If you use a Windows PC and iTunes won't respond or your backup never finishes, you might need to update, change, disable, or uninstall your security software.
4. Remove or Move old backups
   1. Remove corrupted Backup (If you do not need them) --> For Windows PC:- \Users\(username)\AppData\Roaming\Apple Computer\MobileSync\Backup\ (Delete the files here)
   2. If you need copies of your backups, you can find the Backup folder and copy the entire folder (not just part of it) to another place, like an external drive or network storage location.

Try after each step...

1. Update to latest iOS and macOS (You said you have done this)
2. Restart iPhone and Restart Mac.
3. If the FireWall is blocking iTunes, it will not permit to backup. To check it, you should:
   1. Go to System Preferences.
   2. Security (or Security & Privacy) icon
   3. Go to the Firewall tab Turn off Firewall or Ensure iTunes is not blocked by Firewall 
4. Delete iTunes .plist file
   1. Open Finder.
   2. Hold Alt/Option and click on Go.
   3. Click “Library”. (This will appear only if you hold on to Option key on the Keyboard)
   4. Locate com.apple.iTunes.plist file and delete it.
   5. Restart to finish the process.