To use vSphere Web Client, you have to firstly register a vCenter server to it. Otherwise you will see such error messages.
The typical way to register vCenter with vSphere Web Client is through https://<vSphere-web-client-url>:9443/admin-app. But sometimes it may not work, such as you don’t have Adobe Flash Player installed or the web server does not respond in a timely fashion. The alternative way to do it is to run a command line.
cd “C:\Program Files\VMware\Infrastructure\vSphere Web Client\Scripts”
admin-cmd.bat register https://<vSphere-web-client-url>:9443/vsphere-client
https://<vcenter-server-url> <username> <password>
Then type ‘A’ to accept the SSL certificate warning if there is one.