cns_ssi: 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/cns_ssi - from package: xrootd-server

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

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