riemann-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 Debian possible default full paths are:

/usr/bin/riemann-client - from package: riemann-c-client

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

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