When trying to enable SSH access to the NSX 6.4.2 USVM, it fails.
VMware NSX for vSphere 6.4.x
This is because the keys required for SSH are not automatically generated and you need to allow traffic through iptables.
This issue is resolved in VMware NSX for vSphere 6.4.4.
Workaround:
Load the keys manually and start SSH on the USVM.