ymplayer: 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/ymplayer - from package: stymulator

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

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