niReg: 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/niReg - from package: openni

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

On Debian possible default full paths are:

/usr/bin/niReg - from package: openni-utils

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

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