popclient: 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/popclient - from package: fetchmail

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

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