rdesktop: 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/rdesktop - from package: rdesktop

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

On Debian possible default full paths are:

/usr/bin/rdesktop - from package: rdesktop

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

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