notmuch: 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 - from package: notmuch

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

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