qmail-tcpok: 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/qmail-tcpok - from package: qmail
/var/lib/qmail/bin/qmail-tcpok - from package: qmail

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

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