The installer information on the recovery server is damaged - 2017 iMac
This topic has been touched on before but none of it applies to my situation.
I am trying to reset a Mac OS Sierra Retina 5K 27 inch 2017 to sell it. I was following video instructions to reboot with Cmd-R use the Disk Utility to erase the Mac HD with MacOS Extended Journaled selected. This cleared the HD.
Then, when I went to reinstall the OS I get the error message "The installer information on the recovery server is damaged" so I can't do anything else. Thousands of people have had this problem. There are no options to proceed.
The support I've reviewed mentions that Sierra is old so file format may have changed, but I can't reformat the HD in the erasing menu to check. I can't restart in Safe Mode as all I get is a Flashing file folder with a question mark. I can restart in CMD-R mode, but face the same questions all over again with no apparent options.
What am I supposed to do?