I can't upgrade my big nur to os Catalina what could be the reason

If you open System Preferences> Software Update do you see the upgrade notice for macOS Monterey.

If you do not see Monterey there your Mac. may not meet the criteria for installing Monterey.

Click on this link,

How to upgrade to macOS Monterey – Apple Support (UK)

macOS Monterey is compatible with the following computers. 

MacBook introduced in 2016 or later

MacBook Air introduced in 2015 or later

MacBook Pro introduced in 2015 or later

Mac mini introduced in 2014 or later

iMac introduced in late 2015 or later

iMac Pro introduced in 2017 or later

Mac Pro introduced in 2013 or later

View the complete list of compatible computers.

If upgrading from macOS Sierra or later, your Mac needs 26 GB of available storage to upgrade. If upgrading from an earlier release, your Mac needs up to 44 GB of available storage.

Click on the  top left of your screen and select About This Mac

this will tell you which model and year your Mac is.

a new macOS version is not an update to the old, it is something you get from app store not from updates

read

How to upgrade to macOS Monterey - Apple Support

Tips Monterey Upgrade 

If they are running macOS Big Sur then they can upgrade to macOS Monterey via the

System Preferences> Software Update mechanism.