Maintainence mode unavailable…anyone know why?

I have a successful, working install of 8.6.4 and am following the “manual upgrade” notes.  It says to put the site into “Maintenance Mode” but when I do I get an error that “The website encountered an unexpected error. Please try again later.”

Anyone know what is going on?  I’m wondering if it’s somehow a permissions thing and I need to make myself a higher prived admin…but haven’t a clue why I’m not already as high as it goes.

Took a close look at other posts and it seems like everyone else’s issues are afterwards.  In my case, something’s preventing the maintenance mode from even showing up.  Is it that I need to install a module for this?  It’s not in core?

Scratching my head.  Thanks in advance for the help!

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