In VCSA 6.0 by default root cant login as root user. First login as administrator and then give administrator permission to root user.
Login vCenter vsphere client login as administrator
Username : “firstname.lastname@example.org”
Password (root) : “Vmware@123” (example)
Right click vCenter –> Add permission –> role(administrator) user (root) —>ok
then try to login as root user now it will work :):)