yakuake: 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/yakuake - from package: yakuake

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

On Debian possible default full paths are:

/usr/bin/yakuake - from package: yakuake

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

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