pt-mext: command not found

Have You tried accessing the binary with the full path?

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-mext - from package: percona-toolkit

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