I managed to figure it out by myself. Actually it is permission issue.

The ffmpeg command needs to be run with root privilege. Either launching the command with root account, or launching with sudo should fix the problem.

I'm using nanopi-neo-air_FriendlyCore-Xenial_3.4.39_20171102.img as is, without any apt-get update or upgrade, meaning - drivers are provided by friendlycore. - ffmpeg is the friendlycore customized version. I followed the wiki page and run the following command ffmpeg -t 30 -f v4l2 -channel 0 -vide...