This issue is resolved. Thank you!
I had missed the step of going to the screen of Jetson AGX Xavier to set up my account and password. After I configured my account, I was able to install the other SDK packages such as CUDA and completed without errors.
I did not need to configure any IP address manually. The default IPs worked out fine.
Meanwhile, I first connected Jetson AGX Xavier by HDMI/VGA cable to a monitor but it could not work. I saw some text one the screen but it stuck and not entered the Linux configuration menu. That is why I was not aware of the account configuration step at the first place and mistakenly thought I could use nvidia/nvidia for both account and password. After reading your reply and check another post https://forums.developer.nvidia.com/t/problem-flashing-and-booting-xavier/75797 , I tried HDMI/DVI cable with a newer and larger monitor and it worked.
I need to clarify that my HDMI/VGA cable and monitor is not faulty and I have been using it for Jetson Nano. It just cannot work with Jetson AGX Xavier. If I had used the HDMI/DVI cable with that larger monitor at the first place, I probably would not have encountered this issue.