webmitm: command not found

Have You tried accessing the binary with the full path?

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/webmitm - from package: dsniff

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

On Debian possible default full paths are:

/usr/sbin/webmitm - from package: dsniff

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