convert.pyo: 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 CentOS possible default full paths are:

/usr/lib/mailman/bin/convert.pyo - from package: mailman

You may install the required package with command yum install {package}

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