talk: 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 CentOS possible default full paths are:

/usr/bin/talk - from package: talk

You may install the required package with command yum install {package}

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