@jack85_886088460013698 Was fighting this one for quite a while myself, tried every channel that I knew had a camera on it, but didn't bother trying the ones I knew *didn't*.
Turns out the channel-to-camera mapping in the API is different from the one you see in the app and on the HDMI interface, so at least in my case, I just never ended up giving it a correct channel number argument.
You can do a `Getchannelstatus` call to return a list of the *actual* mapping between channel numbers and device names - these are in turn the channel numbers you should use for the NvrDownload call