Wow!! Thank you thank you thank you Javier!
I followed the link - now everything works!
To help anyone else and save you following a link, here are the instructions as copied:
Here's the fix, being broken down dummy style:
1) Download Nova Launcher & set it as default to replace TouchWiz.
2) Long press on the home screen.
3) Choose Widgets.
4) At the top, under Nova Launcher, scroll to the right & press & hold on Activities and place it somewhere empty on the home screen.
5) Scroll down to Setup Wizard. I saw 2 Setup Wizard options.
6) Under the first Setup Wizard option, I opened it & scrolled down to the 3rd choice which said ".SetupWizardTestActivity". It might be in a different spot for you, but look at the small writing, that's how I saw this one.
7) Select that to create the widget shortcut.
8) Open it. Apparently you're supposed to have a menu to go through and just click through "Next", but there wasn't one for me.
The screen just flashed and then next thing you know, everything works!
At 8, my phone took me through a whole lot of setup screens and updated some apps. Just follow them.
The only thing that didn't work then was the linking to the computer.
To fix that, see this video:
USB Connection to PC Problem NOW FIXED in Android 6.0 Marshmallow !!IT Support Services UK
And now everything works!