sisinstall: 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/sisinstall - from package: plptools

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

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