So have you flashed the board?
Flash the board is like installing the OS to your board. Install sdk is just like install some software. OS is needed first so that you can install sdk.
If the board is really flashed correctly, you shall be able to do some basic operation on your jetson.
I remember you were trying to flash your custom board right? And you also told me you can install the sdk already.
Why you asked same question again? Anything changed here?
You already told us →
And I installed development package(CUDA, TensorRT etc…) via SDK Manager.