You apparently don't have permission to access the video device as your user.
The best way to fix this is to put yourself into whatever group owns that devices. A quick and dirty way to do this is with this command:
groupadd -A $(whoami) $(stat -c %G /dev/video0) (adds your user to whatever group owns that devices node). On my system that would be video, but beware that if that devices is owned by root, you end up with your user in the root group (not advised!).
The not so good way to do this but that would work if your distro doesn't have a sensible group ownership of that device node is to open up the device to be read and written by all users with something like
sudo chmod 777 /dev/video0.