why can't I upgrade my imac 2011 to mac0s high sierra?

How to get Old versions of macOS Open the provided link in Safari. From there use Safari again ( FF / Chrome ) will not work) to download the Full Installer of High Sierra. Before doing so, make sure to do a Time Machine Backup and if possible a Bootable Clone to external drive. Insurance incase ti upgrade goes sideways.

... or via Combo-Update

Download macOS High Sierra 10.13.6 Combo Update (apple.com)

Hi,

it should work, via App Store ...

Connecting to the Mac App Store (apple.com)