ModemManager: 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 Debian possible default full paths are:

/usr/sbin/ModemManager - from package: ModemManager

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

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