mdquery: 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/mdquery - from package: shibboleth-sp2-utils

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

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