In the vSphere Client -> Administration -> Single Sign On -> Users and Groups, when you select "localos" as the domain, the "ADD" button is greyed out.
vCenter 7.0
vCenter 8.0
This is a known issue with the vSphere Client and will be fixed in a later release.
Currently, there is no solution to this issue. To workaround this issue, manage localos users via the Appliance Shell.
For further information on managing users using the Appliance Shell, see Managing Local User Accounts in vCenter Server
If the vCenter is logging in to the BASH Shell by default, please see Switching the vCenter Server Appliance 6.x to Appliance Shell