Did you complete the first boot setup? Normally the flash takes place first in recovery mode, and when actual flash completes, then the unit reboots. Then the end user has to create an admin account login, and only then can it go on to installing items via apt (it uses ssh to the admin account, but if the account does not exist, or if the password has not been set up, then this will fail).
Additionally you need internet access. Without this apt can’t succeed.
Do keep in mind that you don’t have to flash every time, you can uncheck flash and just install CUDA if the Jetson is fully booted and has the ability for login via your admin account. Looks like you used name “root123”, and so the test is to see if your host PC (or local at the Jetson) can log in to that account over ssh.
This is the error i get when i try to execute sudo apt-get update && sudo apt-get check
root123@root123-desktop:~$ sudo apt-get update && sudo apt-get check
[sudo] password for root123:
Ign:1 https://repo.download.nvidia.com/jetson/common r32.5 InRelease
Ign:2 https://repo.download.nvidia.com/jetson/t194 r32.5 InRelease
Hit:3 http://ports.ubuntu.com/ubuntu-ports bionic InRelease
Err:4 https://repo.download.nvidia.com/jetson/common r32.5 Release
Could not handshake: Error in the pull function. [IP: 184.25.239.72 443]
Get:5 http://ports.ubuntu.com/ubuntu-ports bionic-updates InRelease [88,7 kB]
Err:6 https://repo.download.nvidia.com/jetson/t194 r32.5 Release
Could not handshake: Error in the pull function. [IP: 184.25.239.72 443]
Get:7 http://ports.ubuntu.com/ubuntu-ports bionic-backports InRelease [74,6 kB]
Get:8 http://ports.ubuntu.com/ubuntu-ports bionic-security InRelease [88,7 kB]
Get:9 http://ports.ubuntu.com/ubuntu-ports bionic-updates/main arm64 DEP-11 Metadata [288 kB]
Get:10 http://ports.ubuntu.com/ubuntu-ports bionic-updates/universe arm64 DEP-11 Metadata [282 kB]
Get:11 http://ports.ubuntu.com/ubuntu-ports bionic-backports/universe arm64 DEP-11 Metadata [9.272 B]
Get:12 http://ports.ubuntu.com/ubuntu-ports bionic-security/main arm64 DEP-11 Metadata [42,3 kB]
Get:13 http://ports.ubuntu.com/ubuntu-ports bionic-security/universe arm64 DEP-11 Metadata [52,8 kB]
Reading package lists... Done
E: The repository 'https://repo.download.nvidia.com/jetson/common r32.5 Release' does not have a Release file.
N: Updating from such a repository can't be done securely, and is therefore disabled by default.
N: See apt-secure(8) manpage for repository creation and user configuration details.
E: The repository 'https://repo.download.nvidia.com/jetson/t194 r32.5 Release' does not have a Release file.
N: Updating from such a repository can't be done securely, and is therefore disabled by default.
N: See apt-secure(8) manpage for repository creation and user configuration details.
