instant-clean: 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/instant-clean - from package: python-instant

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

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