mk-upgrade: 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:

/usr/bin/mk-upgrade - from package: maatkit

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

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