How to turn on camera for FaceTime?

is the camera listed in the system profiler?

if so reboot and reset the SMC

Resetting the System Management Controller (SMC) on your Mac - Apple Support

if the problem continue open the terminal and type the following

sudo killall VDCAssistant

in IOS the settings for FaceTime are in General\Setting\FaceTime

you need to enable FaceTime, there are no other camera restrictions AFAICT.

then proceed as I described in the 2nd post.