popcon-largest-unused: 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/popcon-largest-unused - from package: popularity-contest

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

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