rds-ping: 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/rds-ping - from package: rds-tools

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

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