gdaladdo: 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/gdaladdo - from package: gdal

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

On Debian possible default full paths are:

/usr/bin/gdaladdo - from package: gdal-bin

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

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