Mac OS 11 (Big Sur) Sierra Client

Never use the mac installer from the DVD.  Always download the mac client from the Xitron website:

Once you download the client, launch the .dmg file, which will mount a disk image.  Copy the client into your application folder.

Upon the initial launch of the client, the OS will try to verify the application and its origins.   Likely it will not launch.  Go directly to your System Preferences > Security & Privacy, and click on the “general” tab.  At the bottom, under “Allow applications downloaded from” you will see an alert asking about the Sierra client.  Allow Sierra Client to launch, and you should have no more trouble on that machine.