I would like to add some clarity since this frustrating error could use a lot less of our time trying to get around it.
I have gathered the solution that worked for me from multiple forums beginning with the answer here from Gwopoppam, who started me on the path to success.
In Gwopoppa's instructions, skip the first step of downloading the High Sierra dmg file. You can do it from within the patcher app in step 2 and the patcher won't verify other downloads (or didn't for my 6 versions.)
Insert a Flash Drive (honestly 16GB minimum) and use disk utility to format MacOS Extended (Journaled) & GUID and name the drive MyVolume
Get that patcher app which will either open right up (on my mac mini) or require allowing it via Sys Pref-Security (my 2012 MBP and 2017 MBP TouchBar) where you will instruct your mac to open the file.
From another forum: when the patcher opens up and asks you to click the image to select a file, click Tools in the menu bar which will give one option of downloading the dmg right from apple. This method took 5 minutes as opposed to the 2 hours I waited for 2 different downloads that couldn't be verified by the patcher. When complete it will verify it and ask if you want to create the bootable drive. You don't because it will likely fail while making a drive that looks like it has the right contents. All you want is that download which you are to now copy from the downloads folder (the Install macOS High Sierra app) and paste into the Applications folder.
Open Terminal and follow the Apple link for the terminal commmand to create a High SIerra bootable installer flash drive:
Create a bootable installer - Apple Support
Here's the command and please note naming the drive MyVolume as well as copying the app into the applications folder helps this command work:
sudo /Applications/Install\ macOS\ High\ Sierra.app/Contents/Resources/createinstallmedia --volume /Volumes/MyVolume
After pasting the command in terminal enter your admin password (nothing will appear while typing) and press enter. The bootable drive will be created (8 min)
When it's finished eject the flash drive and remove it.
On the mac you are installing High SIerra on, insert the flash drive and power on holding the option key. Select the drive you just created. It will likely boot to the macOS Utilities screen. If it boots to the installer congratulations, but don't waste time clicking Install MacOS from the utilities menu if you get to that screen.
Disconnect any ethernet cable and/or turn off Wi-Fi.
Open disk utility and format the drive to install the OS on as MacOS Extended (Journaled) and GUID.
Go to Utilities in the menu bar and open Terminal.
Type Date 0101010118 and press enter.
Then type csrutil disable and press enter to turn off SIP.
Click the Apple icon and Restart. Just let it completely restart which should bring you back to the MacOS Utilities screen.
Restart the computer holding the Option Key. Select the bootable flash drive and follow the installation instructions.
Crazy fun!!!! 2 weeks I spent on this iMac. So glad I can hand it back to the owner now.