This one took me a minute to figure out. I had worked on a child theme in WordPress, but ran into the following error when I tried to use Live Preview:
Twenty Seventeen requires at least WordPress version 4.7. You are running version 4.5.20. Please upgrade and try again.
The strange thing is the version of WordPress version 5.x not 4.5.20. Obviously there was a version conflict somewhere. Googling around produced no hits, that made sense.
The cause was that the version of WordPress on the production site was behind the version on the development site. Who knew? So, I updated the production site to the latest version and the problem went away.
I’m posting this so that it can help others.