ca-certreq-mail.py: 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/sbin/ca-certreq-mail.py - from package: pyca

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

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