When this error occurs, try the following:
1. In the Enforce UI, select Administration > System Overview > Detection Server > Configure
a. Check to see if the NIC shows up
b. Check to make sure the correct NIC is selected
2. Open Wireshark to make sure the same NIC key that is used for testing is used on the selected NIC in the Enforce UI
For further troubleshooting, see:
KB 42088: The NIC Does Not Appear in the UI