handler.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/python2.6/site-packages/viewvc/bin/mod_python/handler.pyo - from package: viewvc

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

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