doxygen: 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/doxygen - from package: doxygen

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

On Debian possible default full paths are:

/usr/bin/doxygen - from package: doxygen

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

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