After upgrading from vShield Manager 5.5.4.x to NSX Manager 6.2.4 I logged in via  my own own username (mick@vsphere.local). Upon clicking on the Network & Security I was presented with the following error:

 

To fix the issue log into vcenter using a user with relevant permissions (in this case administrator@vsphere.local)

1) Click Network & Security -> NSX Managers -> Manage -> Users -> {Plus button}
2) Type in the user you want under Specify a vCenter user and then select  ->
3) -> Next -> {choose relevant NSX permissions} -> Next  -> Finish

 

As you'll see from the following screenshot I have already added mick@vsphere.local