yap: 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/yap - from package: yap

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

On Debian possible default full paths are:

/usr/bin/yap - from package: yap

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

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