Hi Thomas,
Not sure if you are still facing the issue, however wanted to share our way of resolving it. As @TomMerhige pointed out, our deployment had allowedProxyHosts restriction applied which, in fact, included Hosting ArcGIS Server machine name. However, "Create Feature Layer" option was still missing. SSL certificates were all valid and trusted across entire deployment.
When running network trace on clicking Create button, we noticed 400 response with Invalid URL error referencing Portals' sharing proxy call to ArcGIS Hosting Server:6443 url.
To resolve: Even though allowedProxyHosts had hosting Server's entry included, we added it again copying the value from the failed request in Network trace: lea-agshosting.services.esriaustralia.com.au
Hope this helps!
Irina.