If you are using a MacOS system and have never used Blackmagic Design products, you will need to allow Mac to recognize them.
Go to System Preferences and open Blackmagic Desktop Video. You may see the window shown below; this is an essential step to allowing Blackmagic device access.
To resolve this, open Security & Privacy from the System Preferences menu.
There will be a message indicating that software from Blackmagic Design was blocked; select Allow on the right side to allow access.
Open the Blackmagic Desktop Video preference again; you should now see your device appear.