xnightview: 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/xnightview - from package: nightview-gui

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

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