yardoc: 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/yardoc - from package: yard

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

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