rclock: 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/rclock - from package: rxvt

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

On Debian possible default full paths are:

/usr/bin/rclock - from package: rxvt

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

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