rhythmbox-client: 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/rhythmbox-client - from package: rhythmbox

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

On Debian possible default full paths are:

/usr/bin/rhythmbox-client - from package: rhythmbox

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

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