You see an alert in the vCenter Server similar to:
Could not establish communication with NSX Manager. Please contact administrator.
VMware NSX for vSphere 6.4.x
To resolve this issue if you are using Active Directory:
Domain information has to be added to the following:
vSphere Web Client > Networking & Security > NSX Manager > Domains.
For more information, see the Register a Windows Domain with NSX Manager section in the NSX for vSphere Administration Guide.