Can’t Download Big Sur

Which exact Mac do you have?

Have you downloaded from here?

macappstores://apps.apple.com/app/macos-big-sur/id1526878132?mt=12

Then Created a bootable installer for macOS...

Create a bootable installer for macOS - Apple Support

There are factors:

Some which are beyond your control and those with are totally under your control

Like poor Wifi connection

Slow wifi connection

Software on the computer interfering with the download process including AntiVirus Software

Notation: Disabling the AV Software may or may not solve the issue.

Removal of the AV Software may increase the chances of the download to perform as expected.

Running behind a VPN which may increase Packet losses or corruption of the download 

Turn off any Third Party Two Way Firewall and try again 

Best Results are via Ethernet Cable to Router

Restart in Safe Mode. This will perform a Disk Repair, clear cache files and only load Apple Software, extensions and fonts. The boot up will be slow and can take some time - Normal 

Worst case - Restart in Recovery Mode and choose Reinstall macOS. 

This in theory will pull the latest version of macOS this computer qualifies to run 

Thank you

An aside

From the time of my posting at 04:33 to now 05:21 almost 1 hour later

The Auto Notification email of your kindness was received

Ouch

I love getting a great helper such as you in my threads!👍🏽

Too kind