Software Update does not show any upgrades for High Sierra
Attempting to upgrade an iMac on High Sierra 10.13.6 (Retina 4K, 21.5 inch, 2017) to Monterey but Software Update does not show any available upgrades. According to the hardware requirements I should be able to upgrade to at least Big Sur, or preferably Monterey.
Why is Software Update not showing upgrades?
iMac 21.5″ 4K, macOS 10.13