//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.