classad_version_ns: command not found

Have You tried accessing the binary with the full path?

Maybe Your $PATH variable doesn't contain the default paths? echo $PATH should give something like:

[root@server ~]# echo $PATH

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/classad_version_ns - from package: classads-devel

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