Yes, That's what MrI pointed out above. It seems NVENC does not work with GeForce GTX (gaming) graphics cards.
Quadro or Tesla gpu's are required.
"its support in official documents"
CUDA, yes. GeForce, no. Apparently there's more to the requirements than CUDA.
https://developer.nvidia.com/nvidia-video-codec-sdk
There is also a special developers forum which may be helpful -
https://devtalk.nvidia.com/
Yes,nvenc works with gtx cards.
https://www.youtube.com/watch?v=Lejul04cMmk
https://www.youtube.com/watch?v=GiycrvlbJSo
But video converters only use cuda, far as I know.