cannot get macos big sur 11.6.8

May yes and maybe not.

If the user would like to dig deeper regarding the overall situation of this computer >>

Understand, we are all working Remotely from all around the world. 

Therefore, we do not have the On-Hands experience the User ( you ) has with this computer.

The next best thing for us to having a semi - actual On-Hand experience on this computer is to follow the steps below

Download the Application Etrecheck directly from a well Respected ASC Contributor. And Safe to use.

The application is free or paid from added features. 

Run the application with Full Disc Access ( Security & Privacy - Full Disc Access ).

It will take a Snap Shot -  both the hardware and software.

 The Report will Not Reveal Any Personal Information. 

Post back the Full Report - copy and paste - using the Additional Text Icon ( 3rd Icon to last )

We can have a look at the report for possible issues and may have possible suggestions to resolve the issues.

Does the intended computer qualify for the upgrade to Big Sur ?

There are factors:

1 - Like poor Wifi connection

2 - Slow wifi connection

3 - Too many other people downloading at the same time and Apple Severs are overloaded

4 - 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.

4A - Any Third Party Applications that will interfere with the normal operation of the OS,  is an invitation for disaster and comprising the Operating System

Any of the below should be removed as per Developers Instructions 

This will include CleanMyMac

 MacKeeper

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

6 - If via wifi too many Other devices using the wifi at the same time reducing the Bandwidth to your computer. 

7 - Best Results are via Ethernet Cable to Router

8 -  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.

Then try again

Part of the Apple Support Community services done on a volunteer basis and " from users like you " Humour here