Preview 10.1 in macOS Mojave does not capture signature with camera
Solution 1:
When you select the camera on the Preview.app, It should open the camera up. You need keep some distance between the FaceTime camera and the white paper only then Preview.app will pick it up. If it's kept too close, the signature will not be picked up.
Solution 2:
It was not working with me in the beginning, but the problem solved when I had a spot light points in parallel between the laptop screen and the paper. Max light is needed apparently.