Hello, whenever I try to upgrade my Mac to Monterey OS it gets stuck at "Checking "null"". Can you help me with that?

In addition to the above I would delete any existing Monterey Installer, which are in the Applications folder. Also remove any anti-virus apps since they care unnecessary and sometime prevent updating. Then boot to Safe Mode, which takes minutes, and download the Monterey installer and run it while in Safe mode.

How to use safe mode on your Mac - Apple Support

Can your mac upgrade to macOS Monterey..

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.

When upgrading your mac should be connected to your router via cable rather than WiFi.

If you can only use WiFi then move your mac as close to your router as possible.

Do not use the mac while it is downloading and do not use any other devices that share the same internet

connection.