This may be related to which TLS versions are enabled for the user running the ArcGIS Server service. It might be worth trying the following:
1) Hold Shift and right-click on the Internet Explorer icon, then choose "Run as different account". Enter the account that is running ArcGIS Server.
2) Go into the Internet Options > Advanced and confirm that TLS 1.1 and TLS 1.2 are both checked (those are near the bottom of the list)
3) Restart the ArcGIS Server service and try to create the site again
Also, while you have Internet Explorer open as that user, confirm you can hit a website on the internet, to confirm that this user has access to make calls out over http/https.
-Danny