mk-query-digest: 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-query-digest - from package: maatkit

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

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