Why I can't install Mojave in my iMac released in Mid 2011

Your iMac is not supported under Mojave. There are ways of getting Mojave onto unsupported Macs but in your case with your iMac this would render it almost unusable, this is due to the graphics card that is installed in it.

To get Mojave buy a newer iMac.

Currently there is no hack or way to upgrade pre 2012 macs to Mojave. Hacks are usually created however Apple has still not released the open source for Mojave and it remains unclear wether they will. As for upgrading the GPU this is possible however I am not sure if a metal supporting GPU can be added. The 2011 iMacs are the last macs where the GPU is not soldered to the board. For more info a YouTube by the name you Luke Miami is attempting to replace the stock you in the 27” 2011 iMac with a 980m from nVidiea. If you really want to get Mojave running you could maybe do it with an EGPU but it would most likely not work or be very unstable.

unfortunately not. you will need to upgrade to a 2012 iMac or later to be able to use Mojave

whether or not your iMac supports metal graphics is dependant on the hardware.

Is it possible to upgrade my iMac to support Metal graphics?

No. Sorry.

When I go to the Mojave page at the App Store, there's no "Get" button---and I see no way to download the new OS.

Any help ? Steve

Because your Mac doesn't support Metal graphics.