Let me do some research in Japan and I get back to you.
Koshian is not designed for software development.
And I recommend to use BCM92073X_LE_KIT for it including viewing debug trace.
I can see debug trace from HCI UART with any sample application on LE_KIT easily.
So please try running existing sample like hello_sensor on LE_KIT first.
If you see the debug trace from HCI UART, then the debug trace will work on Koshian I think.
Thank you all for reply.
I understood koshian is not suitable for software development.
I should use PUART for debug trace because I don't have BCM92073X_LE_KIT.