iMac late 2013, cant upgrade to Big Sur

Here's Apple's own documentation:

macOS Big Sur is compatible with these computers -

https://support.apple.com/HT211238

2015 MacBook

Late 2013 MacBook Pro

Mid 2014 iMac

2013 Mac Pro

2013 MacBook Air

2014 Mac mini

2017 iMac Pro

What do you see when you go to  Menu→About this Mac and click Software Update?

BTW, you posted in the iPhone forum. I’ll ask the hosts to move your post to the iMac forum.