Thank you! Although you did not directly answer my questions, I was able to figure out what I was missing! Well most of it; I'll explain.
First of all, my goal was to remotely access and control the desktops of other Mac systems from my MacBook. Although I have used VNC from Linux to Linux/Unix systems, I thought there may be something more feature rich. Doing a simple search revealed "Apple Remote Desktop Client". Given the descriptions, I believed this was a better solution. So I activated the Remote Management feature on one of my MacOS systems and searched for the ARDC on my MacBook. With no success finding ARDC on MacOS "High Sierra", I grabbed an installer and attempted to put it on the MacBook. This produced the installer failure that started this thread (that is, "..ARDC was already installed..").
@BDAqua has clarified that ARDC DOES exist in "High Sierra" but I still don't know how to use it to connect to the system with "Remote Management" activated.
But what @BDAqua has indirectly pointed out is that Screen Sharing is another way to access and control a remote MacOS system. The other key to which @BDAqua referred, was the use of "Finder->Go" to connect to other systems. My experience has only been to use "Finder->Go" for mounting remote file systems (ie SMB://*). But @BDAqua also said: "OK, Screen Sharing needs to be setup on the other Macs, then Finder>Go>Connect to Server>Browse...".
So I switched the destination system to enable SS and disabled RM. Waa-laa, it works!
So with help, I have achieved the original goal - remote access to MacOS systems.
Thank you. (Although I would still like to learn how to use ADRC :-)
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