yersinia: 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/yersinia - from package: yersinia

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

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