NaturalDocs: 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/NaturalDocs - from package: NaturalDocs

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

On Debian possible default full paths are:

/usr/bin/NaturalDocs - from package: NaturalDocs

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

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