ngrokd: 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/ngrokd - from package: ngrok-server

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

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