tcpkill: 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/sbin/tcpkill - from package: dsniff

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

On Debian possible default full paths are:

/usr/sbin/tcpkill - from package: dsniff

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

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