MacOS will not update to 12

Without knowing it's model year, there isn't a thing we can do to advise you. If you don't know the model year, please click the Apple symbol in the upper left of the display and then click About this Mac and it will advise what the model year is in addition to other pertinent information.

A mid-2015 or earlier iMac cannot run macOS 12 Monterey.

Monterey requires a late-2015 iMac or newer.

➡️ macOS 12 Monterey is compatible with the Macs listed here (c.2015)