shutdown iMac / hold option/command & R keys down on keyboard & restart the iMac.
go into MacOS Utilities / select Disk Utility / select the top HDD in the list on the left / CLick ERASE / then SHUTDOWN again / (repeat the same startup process) hold option/command & R keys down on keyboard & Turn on the power to the iMac WHILE holding the keys down until the MacOS Utilities comes back up. NEXT / select REINSTALL MacOS /continue… then follow the prompts and BE SURE to select the same HDD that you previously formatted before you restarted the iMac. .. this will download the new Catalina OS .. it will take a while… I just did this myself while on the phone with Apple Support… and it worked.
P.S. it's best if your iMac is plugged into a network jack and not on wifi .. but wifi will work just as well but a bit slower.
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