Smart Licensing needs to be enabled on Firepower Management Center (FMC) in order to support Firepower Threat Defense (FTD) sensor. Once Smart License mode is enabled you may notice that your Classic License option disappeared from device configuration screen. Most likely you still have a need to add a classic device, but not sure what to do in this case.
Cisco Firepower Threat Defense Software-6.0.1
First, make sure you have available classic licenses under System > Licenses > Classic Licenses. In the example below I’ve 2 available Protection licenses.
If you were used to assigning licenses to devices during registration, it will not be the case anymore. Now you will Register device without assigning any licenses.
Once registered, you will need to go back to Devices > Device Management > Select Device > Device > Licenses > Edit License and assign license capabilities. Checkboxes will be grayed out if no licenses available.
After assigning proper licenses proceed with device configuration and policy assignment.