Are you able to run the live image recognition (imagenet-camera) first? Or do you only have problem with detectnet-camera?
Also, to check there are no problems with camera feed, please try running nvgstcapture viewer beforehand.
If the board powers off, try running “sudo nvpmodel -m 1” to set it in 5W mode. If it doesn’t shut down any more after that, this would point to a power supply issue.