Hi,
I guess you are new to jetson platform, so better reading this page.
https://elinux.org/Jetson/General_debug
After you build the kernel image, you have to copy it to Linux_for_Tegra/kernel and use the flash.sh to flash.
Do not use sdkmanager anymore because it might download a kernel image from server again and overwrite your work.
For me, I only use sdkmanager once to download the Linux_for_Tegra and never use it anymore unless I want a clean package.