Here's how to turn on the on-screen keyboard in macOS Sierra:
1. Go to System Preferences > Keyboard.
2. Check the box that says "Show keyboard and emoji viewers in menu bar."
3. Click the icon in the Menu Bar with the Command (⌘) symbol in it, and choose "Show Keyboard Viewer."
If you choose "Show Emoji and Symbols," you'll get a clickable menu of emoji.
4. The keyboard will show up on your screen. You can click on letters to insert them in documents, or watch them become highlighted when you type on the actual keyboard.