modem-manager-gui: 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/bin/modem-manager-gui - from package: modem-manager-gui

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

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