pbuilder-user-mode-linux: 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/pbuilder-user-mode-linux - from package: pbuilder-uml

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

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