s2s: 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/s2s - from package: jabberd

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

Error in other languages:
s2s: bevel nie gevind nie
s2s: comandă negăsită
s2s: comando não encontrado
s2s: commande introuvable
s2s: command not found
s2s: käsku ei ole
s2s: không tìm thấy lệnh
s2s: komanda nerasta
s2s: Kommando nicht gefunden.
s2s: kommandot finns inte
s2s: komut yok
s2s: nie znaleziono polecenia
s2s: níor aimsíodh an t-ordú
s2s: no se encontró la orden
s2s: no s'ha trobat l'ordre
s2s: opdracht niet gevonden
s2s: parancs nem található
s2s: perintah tidak ditemukan
s2s: príkaz nenájdený
s2s: příkaz nenalezen
s2s: команда не найдена
s2s: командата не е открита
s2s: 命令找不到
s2s: コマンドが見つかりません