Are you sure that the user Administrator have full privileges in that vCenter? Same behavior happens if you try to use the SSO user administrator@vsphere.local? Not that the local or domain does not necessarily have full permissions on the vCenter Server.
Thanks for your input Richardson Porto,
I'm using with same SSO user administrator@vsphere.local, my ESXi 6.0 may be over 60 days of evaluation mode, hence it could issue..? I guess..
This issue also could be due to incorrect root password of ESXi.
But unfortunately it is not so easy to recover the ESXi password and recommended ways are published in below article.
Changing a forgotten root password on an ESX/ESXi host (1317898) | VMware KB
Regards
Lokesh