NiUserTracker: 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/NiUserTracker - from package: openni-examples

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

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