clean_conntrack.pl: 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/sbin/clean_conntrack.pl - from package: nuauth-utils

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

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