yui-compressor: 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/yui-compressor - from package: yui-compressor

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

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