Trying to downgrade from Monterey to Big Sur - followed instructions but message pops up and says this volume is not down gradable???

It sounds like you are trying to run the Big Sur installer on the Monterey startup disk. You have to first erase the Monterey startup disk and then install Big Sur on that disk. Then you restore stuff from your ™ backup.

Below method will WIPE ALL DATA and There is No 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.

You may want to try holding down the Option key to launch the Startup Manager and then choose the Install Big Sur USB. After installing Big Sur, you can use the Migration Assistant to recover files from Time Machine. Be careful to choose either a Time Machine backup with Big Sur or transfer user files only (and perhaps third party applications) from a backup made with Monterey (system settings and some configuration will need to be redone manually).