If you go to Settings -> "Biometrics and security" -> "Other security settings" -> "Device admin apps" you can see which apps may be blocking the use of the camera.
I just got this problem today right after I added two fingerprints. I tried your suggestion above. The only two apps I see under Device admin apps is Find My Device and Google Pay. Neither of those seem to be causing the problem. Help!
If the settings didn't work that longhiker provided above, I would recommend trying to put your device in safe mode, to see if you can access the camera without the error. By putting it in safe mode, this will let us know if it's a third party app that is causing the symptoms you're experiencing.
If the message still occurs in Safe Mode, service would be recommended. Please send a private message with the full model code and the IMEI number of the phone. http://bit.ly/2XERsky