macOS Big Sur could not be verified

The error message may be misleading. Suggest checking in Applications Folder and look for a New Application called Install Big Sur. If there is one by this name Drag And Drop to Trash and Empty Trash.

Connect via Ethernet Cable to Route for best results. If via Wifi - disconnect ALL Other devices using the Wifi - what the Maximum Bandwidth only for this computer.

Restart in Safe Mode - Intel Version is SHIFT immediately at startup. This does a repair Disk, clears cache files and only loads Apple Software , extensions and Fonts. Usually takes some time to load - Normal. Once logged into the computer - Open the Apple Apps Store and find Big Sur. Download the Full Installer of about 12.24 GB. There are often several Pauses along the way where count down clock say 15 minutes but in Real Time maybe 45 minutes and so on.

Once the Full Installer is downloaded - suggest stopping there and perform a Time Machine Backup and if possible a Bootable Clone incase things go sideways - Insurance. Also, may consider making a Copy of the Installer Appliation to external drive for future usage like a Bootable Installer

Now and only now - run the Installer

Thanks for the response. Your suggestion did the trick. All is well.