night_darks: 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/bin/night_darks - from package: nightview-cli

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

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