send-reminders: 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/send-reminders - from package: email-reminder

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

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