tcpreen: 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/tcpreen - from package: tcpreen

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

On Debian possible default full paths are:

/usr/bin/tcpreen - from package: tcpreen

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

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