My Mac won't install Big Sur. Why not?

The model and year of your iMac will determine if it can install macOS Big Sur.

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

Read this article.

How to upgrade to macOS Big Sur - Apple Support

macOS Big Sur is compatible with the following computers. 

MacBook introduced in 2015 or later

MacBook Air introduced in 2013 or later

MacBook Pro introduced in late 2013 or later

Mac mini introduced in 2014 or later

iMac introduced in 2014 or later

iMac Pro

Mac Pro introduced in 2013 or later

View the complete list of compatible computers.

If upgrading from macOS Sierra or later, your Mac needs 35.5GB of available storage to upgrade.

If upgrading from an earlier release, your Mac needs up to 44.5GB of available storage. To upgrade from OS X Mountain Lion, first upgrade to OS X El Capitan, then upgrade to macOS Big Sur.

Shopping - Apple does offer a Trade-in Process against the purchase of a New Apple Computer - dependent on your location.

Go to tour County of location for Apple site

In my case, Canada https://www.apple.com/ca/trade-in/. One is be able to commence the process and get an Estimate for existing units value against the cost of the new machine.

Have used this method several times before and has worked very well.