condor_drain: 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/condor_drain - from package: condor

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

On Debian possible default full paths are:

/usr/bin/condor_drain - from package: htcondor

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

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