This issue is resolved in VMware NSX for vSphere 6.4.0, available at
VMware Downloads.
Workaround:
To work around this issue, clear this message from the GUI is to resync the NSX prepped clusters that use the Distributed Firewall.
To resync the Firewall on at the cluster level from NSX, navigate to:
Network and Security > Installation > Host Preparation > Select a Cluster > Actions > Force Sync Services > Check the "Firewall" box > click OK.