xrdsssadmin: 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/xrdsssadmin - from package: xrootd-server

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

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