TX2 problems :start and display

why don’t my TX2 ask for password when starting?
in addition, it can’t show anything after restart.did I do something wrong?

In my opinion it is a custom option if to ask for a password or not after restart.
For example at “System Settings > User Accounts’ you can toggle the option 'Automatic Login”
But what do you mean saying " it can’t show anything after restart"? Does it load some GUI? Does it throw some errors? Displays blank screen or what?

Do you install it with our Jetpack?

Unfortunately the default for Ubuntu seems to be autologin enabled. JetPack is installing from a sample rootfs set for this. See if what @Andry1984 mentions does the job for you. More details are needed for the questions he asked.