open: 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/open - from package: kbd

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

On Debian possible default full paths are:

/bin/open - from package: kbd

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

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