pt-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 Debian possible default full paths are:

/usr/bin/pt-upgrade - from package: percona-toolkit

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

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