notmuch-mutt: 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/notmuch-mutt - from package: notmuch-mutt

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

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