Upgrading an iMac from High Sierra 10.13.6 to 11

Refer to How to download and install macOS - Apple Support.

As How to update macOS using Software Update explains,

"Software Update shows only software that is compatible with your Mac. If Software Update says that your Mac is up to date, then no new software is currently available for your Mac model."

The macOS version you can upgrade to depends on the year your Mac was released.

Click the Apple menu () at the top of your screen and choose About This Mac.

➡️ macOS 11 Big Sur is compatible with the Macs listed here (c.2013)

You're welcome. 🙂