Every page that uses this layout now has the chat. You don't need an app from the Shopify App Store.
When you change or update your theme
The code lives in your theme, so a new theme (or a theme update that replaces theme.liquid) won't have it. After you switch or update, open theme.liquid again and check that the line is still above </body>. If not, paste it back.
The chat on some pages only
Keep the code in theme.liquid, then list the pages that shouldn't show the chat under Chat Widget › Behavior › Pages without the chat, for example /account/* or /policies/*.
If the chat doesn't show on Shopify
You edited a different theme. Only the theme marked Current is what visitors see. Add the code to that one.
Your store is password-protected. The chat still shows to you once you're past the password page, but our installation check can't get in. Open the store yourself to test.
The code went into a section file. Paste it into layout/theme.liquid, so it's on every page.
Check that it works
Open your site in a private (incognito) window, so you see it the way a visitor does.
Look for the chat button in the bottom corner of the page, then send yourself a test message.
In the dashboard, the message appears in your Inbox as it arrives. Reply, and watch the answer reach the website.
You can also choose check the installation on the Install tab of Administration › Chat Widget: we load your home page and look for your code. No button? Go through when the chat doesn't show.
Set up the rest
The chat is on your Shopify site. A few minutes in the dashboard makes it yours: