latexmk: 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 Debian possible default full paths are:

/usr/bin/latexmk - from package: latexmk

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

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