modes_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/modes_gui - from package: gr-air-modes

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

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