watch_maildirs: 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/watch_maildirs - from package: watch-maildirs

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

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