icinga2: 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/sbin/icinga2 - from package: icinga2-bin

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

Error in other languages:
icinga2: bevel nie gevind nie
icinga2: comandă negăsită
icinga2: comando não encontrado
icinga2: commande introuvable
icinga2: command not found
icinga2: käsku ei ole
icinga2: không tìm thấy lệnh
icinga2: komanda nerasta
icinga2: Kommando nicht gefunden.
icinga2: kommandot finns inte
icinga2: komut yok
icinga2: nie znaleziono polecenia
icinga2: níor aimsíodh an t-ordú
icinga2: no se encontró la orden
icinga2: no s'ha trobat l'ordre
icinga2: opdracht niet gevonden
icinga2: parancs nem található
icinga2: perintah tidak ditemukan
icinga2: príkaz nenájdený
icinga2: příkaz nenalezen
icinga2: команда не найдена
icinga2: командата не е открита
icinga2: 命令找不到
icinga2: コマンドが見つかりません