Why the software update cannot be installed?

Hi, elie.fara7.

We were sorry to read that you were experiencing issues with installing the macOS Big Sur update. If you’re still experiencing the issue, we have a couple of suggestions that may help.

First, make sure that the date and time on your Mac are set correctly. Even a slight variation could cause a communication issue.

If the date or time is wrong on your Mac - Apple Support

If it persists, you may need to install macOS Big Sur from Internet Recovery. You can do this by restarting and immediately pressing and holding Option-Command-R. You should see a globe, and it may take a little longer to boot up than you’re accustomed. When you see macOS Utilities, you should see the option to Reinstall macOS and choose macOS Big Sur. This article explains:

Reinstall macOS - Apple Support

Alternatively, you may also be able to install by creating a bootable installer:

How to create a bootable installer for macOS - Apple Support

We appreciate you taking the time to post in the Apple Support Communities.

Warmest Regards.