guake: 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 CentOS possible default full paths are:

/usr/bin/guake - from package: guake

You may install the required package with command yum install {package}

On Debian possible default full paths are:

/usr/bin/guake - from package: guake

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

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