//Situation

    System prompts "xxx is not in the sudoers file"(xxx equals the user name) while executing command "sudo": 

      sudo -i
Password:
xxx is not in the sudoers file. This incident will be reported.

//Analysis

    The current account must have no rights to execute "sudo".

//Solution

To fix the problem, add the current account into the file  "/etc/sudoers":

    1. Switch to supervisor:    su

    2. Open the file "sudoers" with vi editor:    vi /etc/sudoers

    3. Add the account:    press "i" to edit; find line "root    ALL=(ALL)    ALL"; new a line "xxx    ALL=(ALL)    ALL" below it; save&quit with the command "wq!".

Problem solved.