I want to install Big Sur 11.6

As previous posting indicated - the Apple Licence of Use would not allow downgrading to a version of macOS older that the version the computer came with as New. That being, if Monterey was the version as New - that is a far back as this computer could every downgrade.

Otherwise - The " Age of this computer" from Apple Icon on the Desktop would provide the Age of this computer and better identify to what older version it might be able to downgrade to.

Should be qualify for a older version of macOS >>>

The Below will Wipe ALL DATA and there is not recovery - 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 

>> Only works on Intel Based Apple Computers.

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.

Tips How to Upgrade to Big Sur 

There are several points to consider before attempting the upgrade to Big Sur. The most important is Due Diligence on the part of the User and is a Prerequisite for a smooth upgrade.  

The below points should all be consider Before attempting the upgrade.

Point # 1 - Are there pre-existing Unresolved Software Issues. They should be resolved prior to upgrading - they may carryover to Big Sur making the system unstable / unusable.

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 The Built in Security  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.

Point # 3 -  How to get old versions of macOS  ie: Empty Space Requirements specific to the Big Sur. Apple states 35.6 GB Empty Space but suggest 50 GB

Point # 4 - Does the intended computer qualify for the upgrade.

Point # 5 - Are the installed Applications / Extensions compatible with Big Sur. Check with the Developer for compatible Apps / extension or do they offer updates. Same goes for Third Party Drivers for Printer, Scanners etc.

Point # 6 - If upgrading from Mojave and earlier versions of macOS - Remember that Big Sur does not support anything 32 bit. These include Applications, Extensions and Drivers for any devices. There is no workaround.

Point # 6A - To check for 32 bit Applications - a good application to use is Go64 - safe to use.

Point # 7 - Do I have a Rescue Plan In-Place. This comprises having 3 Backups using 2 methods and 1 Off Site incase of Theft, Loss or Natural Disasters. A Time Machine Backup  is very useful and can be used to Revert to Previous Working macOS. For addition purposes - two Tested Bootable Clones each to separate external drives. This is insurance incase the upgrades goes sideways. At least one Tested Clone and / or Time Machine Backup should be Off - Site

Point # 8  - To upgrade to Big Sur will requires a quick, stable and reliable Internet Connection - preferred method is via Ethernet Connection. 

If not available and using Wifi - move computer as close to the Access Point / Router and disconnect all other devices connected to Wifi. Want the maximum bandwidth for the computer being upgraded to Big Sur.

If you really mean 11.6 and not the current version of Big Sur (11.6.2), you have to be running Catalina or later and use Terminal to download 11.6.

softwareupdate --fetch-full-installer --full-installer-version 11.6

If you are running Mojave or earlier, updated to Catalina, then use Terminal in Catalina.

How to get old versions of macOS - Apple Support

If you want the current version of Big Sur, you can get it from the link above (use Safari).

What is your iMac Pro running now? What version of macOS did it come with?

Did your Mac come with macOS Monterey already loaded? If so, you will not be able to downgrade to an earlier version.