nInvaders: 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/nInvaders - from package: ninvaders

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

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