Why does Big Sur update installation failed over and over again?

Hello archrider,

Thanks for asking the Apple Support Communities for help with downloading an installing macOS Big Sur.

1. First, ensure you are connected with an ethernet cord, or on a wireless network that meets these requirements. If the issue continues, you may want to try on another wireless network or with direct ethernet connection: Recommended settings for Wi-Fi routers and access points - Apple Support

2. If you can not direct connect, try moving away from possible wireless interference: Resolve Wi-Fi and Bluetooth issues caused by wireless interference - Apple Support

3. If you still have issues, it may also be beneficial to remove any external drives, hubs, external monitors, printers, etc. while download and installing.

4. Lastly, you might try booting to safe mode to rule out any software conflicting with the installation process: How to use safe mode on your Mac - Apple Support

You can delete any previous downloads, boot into safe mode, redownload the update, and then try to install again, all while in safe mode.

Thanks and have a wonderful day.