hello jack_lan,
please check the sources list, /etc/apt/sources.list.d/nvidia-l4t-apt-source.list. it’ll move to the release version you’ve point to.
currently, 4.6.1 => 5.0 isn’t support since JP5.0 DP is now developer preview version.
are you having the device fused?
could you please also check bootloader logs, please check whether it’s loading via file system or kernel-dtb partition.