what is back to my mac?

Go to Finder. Click the Help menu. Type in "Use Back to my Mac." Click on the help article.

The selected topic is currently unavailable

Make sure you’re connected to the Internet. For help connecting, choose Apple menu > System Preferences, click Network, and click “Assist me.”

If you’re connected to the Internet, and the content still doesn’t appear, try again later.

Here's what I get after doing what you said. I get this from help all the time and trying later never works.

so your answer was of no help to me.