You can preview your website using the live view panel to the right.
Click the play button in the live view panel to preview the current file.
The preview will automatically update after a file is saved.
You can open the preview in it's own window by clicking the pop-out button.
Check the preview url is correct by clicking the popout button and seeing if the page loads.
Confirm your web url is correct in site menu -> Edit site..
If the web url is non-https then the preview may not show. Check if you have a shield in the address bar. If so, click the shield and then "Load unsafe script" and try again.