my computer needs at least iOS 11. where can I get that?

What year model is the iMac?

see > Identify your iMac model - Apple Support

You need to have a Late 2014 iMac for macOS 11 Big Sur and a Mid 2015 iMac for macOS 12 Monterey.

see > How to download and install macOS - Apple Support

macOS Big Sur requires one of these Macs…

macOS Big Sur is compatible with these computers - Apple Support

If that's what you have, get it here: How to download and install macOS

Macs don't run iOS. iPhones run iOS.

Macs run MacOS.

Sorry. MacOS 10.15.7

2015