Big Sur 11,6 Clean Install

Open Terminal and copy and paste without quotation marks " softwareupdate --fetch-full-installer --full-installer-version 11.6 "

This should bring down the full version Big Sur 11.6 and place a New Application in Applications folder called Install Big Sur.

The below will Wipe the Entire Drive of everything - There are no Do Overs - period

To perform this action will require booting from a Bootable Installer. This will have to be performed from a Qualifying Computer to run the version of macOS to be made on the Bootable Installer. Example : Bootable Installer of Big Sur would have to be done on a computer that Qualifies to run Big Sur.

Once that is done read on for preparing the Destination computer  >> Special Notation - works on Intel Based Apple Computers & May on Apple Silicon M1 CPU

Extra Special Notation regarding the Touch ID equipped Apple Computer. About Startup Security Utility and Must Enable from Recovery Mode the ability to boot from External Drive Before Attempting 

1 - Shutdown computer and disconnect all external drive Except the newly created Bootable Installer.

2- Restart and immediately hold the OPTION key until the Startup Manager appears and choose the USB Drive. 

3 - It will present options >> Disk Utilities >> View >> View ALL attached Drives. 

4 - Choose the Upper Most Drive ( not the volumes indented and list below ).

5 - The drive normally is called Apple Media or Apple SSD - that is the drive to Erase and format as APFS with the GUID Partition Map. 

6 - Once that is done >> backup out of Disk Utilities and choose install macOS. 

7 - Follow the prompts and it may automatically reboot several time. 

8 - Upon a final reboot - Setup Assist will present with the newer version of macOS.

MBA M1 here with Big Sur 11.5.2 and ran the Terminal command softwareupdate -l  and listed Big Sur 11.6 available.

Have just run Terminal command  softwareupdate --fetch-full-installer --full-installer-version 11.6 and in the process of downloading at 20% - now 25 %

So the command works.

Running over wifi with No Other devices sharing the Bandwidth or connected to Router

Again in Terminal copy and paste " softwareupdate -l  " without quotation marks.

What are the results.

Am assuming this is all being done from a computer that Qualifies for Big Sur or is already running Big Sur

Well, would expect the Mac Mini would return that response. It is already running the latest Big Sur 11.6. Were you expecting otherwise ??

Ran the " softwareupdate -l " yesterday Tuesday Sept 14 at 10:30 AM EDT on my Mac Mini M1 and reported the 11.6. Then ran the Fetch command at 10:16 AM EDT and brought the Full Version down.

The MacBook Pro with 11.5.2 - that is not expected. No Explanation why, unless, it already has the Full Installer in the Application Folder waiting to be installed.

It may very well be what an "antivirus", "cleaner" or vpn as P. Phillips said.

So that we can look for that kind of problem, I suggest you run Etrecheck and post its report here.

If not, then it may be just a timing thing. Updates to the App Store can appear somewhat gradually depending on regions.

Agree Totally on All Points made by @Luis S above.

Suggest downloading the Application Etrecheck  directly from a Well Respected ASC Contributor. 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.

Any Third Party Applications that will interfere with the normal operation of the OS, alter, modify, remove or delete or attempt to do so is an invitation for disaster and may require a Reinstallation of the OS.

Point # 2 - Are there any AntiVirus, Disk Cleaner, Optimizers, Un-installers, etc installed which should be removed as per Developers Instructions. They are useless, unneeded, cause havoc and interfere with the normal operation of the OS and may even Corrupt the OS requiring a Reinstallation. The Built-in Security of Big Sur is all that is required.

Point # 2 A - Commercial VPNs should only be used if Mandated by Corporate Head Office for working remotely. Otherwise - ones Privacy and Security touted by the VPN is suspect.

Any or all the above will interfere with the Download Process

That specific Question is best asked of Apple.

Only they can answer that question.

These ASC Forums are only Apple Users like yourself and not Apple Proper