This error message is usually caused by a problem with the internet connection or the Apple servers. Here are a few things you can try:
- Check your internet connection and make sure it is stable.
- Try downloading the update at a different time, as Apple servers may be experiencing high traffic.
- Restart your Mac and try downloading the update again.
- Try downloading the update using a different internet connection.
It's also worth mentioning that when you erase your device and use recovery mode, it will erase all data and settings, however it will not remove the Activation Lock that is tied to your Apple ID. So, before selling it, you should sign out of iCloud and the iTunes & App Store, then deactivate Find My Device, this will remove the Activation Lock.
Cheers,
Mark