Hi,
There are related topics:
Jetpack 4.2.1 : building v4l2loopback fails - #5 by Honey_Patouceul
errror executing modules prepare - #2 by linuxdev
implementation of v4l2loopback for devkit onboard csi camera - #8 by _av
Please check and give it a try.
With fakesink it runs well:
$ gst-launch-1.0 videotestsrc ! ‘video/x-raw,width=1280,height=720,framerate=(fraction)30/1’ ! nvjpegenc ! ‘image/jpeg,width=1280,height=720,framerate=(fraction)30/1’ ! fakesink
So if v4l2loopback is installed properly, it should work fine.