yao: 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/yao - from package: yorick-yao

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

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