Q: Preview.app doesn't allow capture of signature

Had this problem too.   In my case "Preview" defaulted to use "Camtwist" as camera source, with no option to choose another.

I uninstalled Camtwist. Now it uses the builtin Camera again.  Check if you have any other webcam .kext you can temporarily disable.

i am using MacOS Sierra and I am getting the same issue with Preview.

When you select Camera in "Create Signature" you get an error dialog: The operation could not be completed.

I fixed this. I found that I could nt load the camera in Facetime.

Re: My FaceTime camera is not working in El Capitan (Thunderbolt Display)

Basically:

- Goto Terminal

- sudo killall VDCAssistant

Then restart your Mac.