knpplayer: 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/knpplayer - from package: kmplayer

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

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