mne: 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/mne - from package: python-mne

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

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