Macintosh HD is a backup drive?

I am trying to upgrade the OS on my mother’s late 2017 27” 5K iMac. She’s still on High Sierra 10.13 and I need to be on MacOS 11 or later to install Office 2021. More to the point, I want her to have all the latest security patches.

ANY upgrade of the OS fails fails because “Macintosh HD is a backup drive”. I suspect this is because these models used a Fusion drive but am open to suggestions.