netkit-ntalk: 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/netkit-ntalk - from package: talk

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

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