pt-sift: 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-sift - from package: percona-toolkit

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