PowerCLI C:\> Connect-NsxServer -vCenterServer vcenter.vinsight.com                                                                                                                                              Using existing PowerCLI connection to 192.168.1.10
Connection to NSX server 192.168.1.10 failed : Invoke-NsxRestMethod : The NSX API response received indicates a failure. 403 : Forbidden : Response Body:
At C:\Users\PranayKJha\Documents\WindowsPowerShell\Modules\PowerNSX\3.0.1174\PowerNSX.psm1:4939 char:13
+             Throw "Connection to NSX server $NsxServer failed : $_"
+             ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    + CategoryInfo          : OperationStopped: (Connection to N...Response Body: :String) [], RuntimeException
    + FullyQualifiedErrorId : Connection to NSX server 10.65.252.134 failed : Invoke-NsxRestMethod : The NSX API response received indicates a failure. 403 : Forbidden : Response Body:

PowerCLI C:\>    

Solution:

Instead of entering vCenter name, enter NSX manager IP. Use admin account when ask for credentials.