ssserver: 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 Debian possible default full paths are:

/usr/bin/ssserver - from package: shadowsocks

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