I have an imac from late 2013, why cant I upgrade to the newest IOS?

Your iMac doesn't meet the minimum hardware requirements macOS - How to Upgrade - Apple

You can run macOS 10.15 Catalina How to get old versions of macOS - Apple Support

Because it is too old. It is not supported anymore.

Cheers,

Jack

I have a 21.5 inch iMac and the IOS is Catalina. Is there an upgrade that I can instal?

Apple has made 21.5 iMacs since late 2009, would you care to elaborate to tell us which year 21.5" iMac you own?