There is a quick fix for the following error:
Runtime extension is not available: use
Cannot serve request to /aem/start.html in /libs/granite/ui/components/shell/header/user/user.html
org.apache.sling.scripting.sightly.SightlyException: Runtime extension is not available: use
You should notice that this occurs when you deploy a new bundle and will appear when you attempt to render anything in AEM or your own custom applications.
Go to http://localhost:4502/system/console/scriptcache and click “Clear Cache” to resolve the issue.