mkdir: command not found

Maybe Your $PATH variable doesn't contain the default paths? echo $PATH should give something like:

[root@server ~]# echo $PATH
/usr/local/sbin:/usr/local/bin:/sbin:/bin:/usr/sbin:/usr/bin:/root/bin

If it's empty, You may set the PATH variable with this command

export PATH="/usr/local/sbin:/usr/local/bin:/sbin:/bin:/usr/sbin:/usr/bin:/root/bin"

On CentOS possible default full paths are:

/bin/mkdir - from package: coreutils

You may install the required package with command yum install {package}

On Debian possible default full paths are:

/bin/mkdir - from package: coreutils
/usr/lib/plan9/bin/mkdir - from package: 9base
/usr/lib/klibc/bin/mkdir - from package: klibc-utils

You may install the required package with command apt-get install {package}

Error in other languages:
mkdir: bevel nie gevind nie
mkdir: comandă negăsită
mkdir: comando não encontrado
mkdir: commande introuvable
mkdir: command not found
mkdir: käsku ei ole
mkdir: không tìm thấy lệnh
mkdir: komanda nerasta
mkdir: Kommando nicht gefunden.
mkdir: kommandot finns inte
mkdir: komut yok
mkdir: nie znaleziono polecenia
mkdir: níor aimsíodh an t-ordú
mkdir: no se encontró la orden
mkdir: no s'ha trobat l'ordre
mkdir: opdracht niet gevonden
mkdir: parancs nem található
mkdir: perintah tidak ditemukan
mkdir: príkaz nenájdený
mkdir: příkaz nenalezen
mkdir: команда не найдена
mkdir: командата не е открита
mkdir: 命令找不到
mkdir: コマンドが見つかりません