Runtime extension is not available: use

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

Exception:
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.