This happens due to requirements in a HA cluster where the management network (that is the service console or VMkernel port for management) is required to have two physical NICs.
If this requirement is not met the above error message is displayed. While I recommend you always, always ensure the management network has two pNICs and is redudant (including via seperate physical switches), it might not be possible in a demo/test environment.
In which case you may want to disable this warning message.
This can be done as follows...
1. Go to "Edit" the cluster settings
2. Click "VMware HA"
3. Click "Advanced Options"
4. Add "das.ignoreRedundantNetWarning" and set the value to "True", Click Ok
5. If there error still shows you need to select the host and click "Reconfigure for HA"
