ss: 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/sbin/ss - from package: iproute
/usr/bin/ss - from package: stripesnoop

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

On Debian possible default full paths are:

/bin/ss - from package: iproute2

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

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