tcpdump: 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/tcpdump - from package: tcpdump

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

On Debian possible default full paths are:

/usr/sbin/tcpdump - from package: tcpdump

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

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