db_upgrade: 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/db_upgrade - from package: db4-utils

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

On Debian possible default full paths are:

/usr/bin/db_upgrade - from package: db-util

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

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