mk-error-log: 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-error-log - from package: maatkit

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

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