It’s not working since April of 2018 because Google decided to give greater control of playback to users. You just need to add &mute=1 to your URL. Autoplay Policy Changes
<iframe id="existing-iframe-example" width="640" height="360" src="https://www.youtube.com/embed/-SFcIUEvNOQ?autoplay=1&mute=1&enablejsapi=1" frameborder="0" style="border: solid 4px #37474F" ></iframe>
Update :
Audio/Video Updates in Chrome 73
Google said : Now that Progressive Web Apps (PWAs) are available on all desktop platforms, we are extending the rule that we had on mobile to desktop: autoplay with sound is now allowed for installed PWAs. Note that it only applies to pages in the scope of the web app manifest. https://developers.google.com/web/updates/2019/02/chrome-73-media-updates#autoplay-pwa