nag-import-vtodos: 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/bin/nag-import-vtodos - from package: php-horde-nag

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

On Debian possible default full paths are:

/usr/bin/nag-import-vtodos - from package: php-horde-nag

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

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