[test@manager ~]$ sudo -l
User test may run the following commands on this host:
    (ALL) ALL
[test@manager ~]$ sudo ls /root
anaconda-ks.cfg  Desktop      install.log.syslog                     l.of             page.html  VTune
cpi              ex           intel                                  mpd.hosts        #te.c#     workspace
cpi.c            ex1.c        libxrdisk_1_BPET46J_02_09_14.so.1.0.0  my_page          te.c       x
deli             install.log  list_disk_space_used_by_users.sh       mysqlaccess.log  test       xCATdb-dump
[test@manager ~]$ sudo useradd myuser
sudo: useradd: command not found
[test@manager ~]$

Probably because useradd isn't in your path and has to be called using the full path (eg /usr/sbin/useradd). The reason simply is that sudo doesn't change the environment in any way, it just performs an ID change.

You are correct.
But if I want to use command sudo cd /root ,
how do I use the full path of command cd?

[test@manager proc]$ whereis cd
cd: /usr/share/man/man1/cd.1.gz /usr/share/man/man1p/cd.1p.gz /usr/share/man/mann/cd.n.gz

That's because cd isn't a program, but a shell-builtin. Always. It wouldn't be useful as a standalone utility, as it can't change the working directory of any other program.

Just edit .profile and find your path and add /sbin , relaunch a shell and try it again.