tcpcat: 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/bin/tcpcat - from package: ucspi-tcp-ipv6
/usr/bin/tcpcat - from package: ucspi-tcp

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

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