ser2net: 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/ser2net - from package: ser2net

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

On Debian possible default full paths are:

/usr/sbin/ser2net - from package: ser2net

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

Error in other languages:
ser2net: bevel nie gevind nie
ser2net: comandă negăsită
ser2net: comando não encontrado
ser2net: commande introuvable
ser2net: command not found
ser2net: käsku ei ole
ser2net: không tìm thấy lệnh
ser2net: komanda nerasta
ser2net: Kommando nicht gefunden.
ser2net: kommandot finns inte
ser2net: komut yok
ser2net: nie znaleziono polecenia
ser2net: níor aimsíodh an t-ordú
ser2net: no se encontró la orden
ser2net: no s'ha trobat l'ordre
ser2net: opdracht niet gevonden
ser2net: parancs nem található
ser2net: perintah tidak ditemukan
ser2net: príkaz nenájdený
ser2net: příkaz nenalezen
ser2net: команда не найдена
ser2net: командата не е открита
ser2net: 命令找不到
ser2net: コマンドが見つかりません