tcpnice: 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/tcpnice - from package: dsniff

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

On Debian possible default full paths are:

/usr/sbin/tcpnice - from package: dsniff

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

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