udisks2_monitor: command not found

Have You tried accessing the binary with the full path?

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/lib/plainbox-providers-1/checkbox/bin/udisks2_monitor - from package: plainbox-provider-checkbox

You may install the required package with command apt-get install {package}
Error in other languages:
udisks2_monitor: bevel nie gevind nie
udisks2_monitor: comandă negăsită
udisks2_monitor: comando não encontrado
udisks2_monitor: commande introuvable
udisks2_monitor: command not found
udisks2_monitor: käsku ei ole
udisks2_monitor: không tìm thấy lệnh
udisks2_monitor: komanda nerasta
udisks2_monitor: Kommando nicht gefunden.
udisks2_monitor: kommandot finns inte
udisks2_monitor: komut yok
udisks2_monitor: nie znaleziono polecenia
udisks2_monitor: níor aimsíodh an t-ordú
udisks2_monitor: no se encontró la orden
udisks2_monitor: no s'ha trobat l'ordre
udisks2_monitor: opdracht niet gevonden
udisks2_monitor: parancs nem található
udisks2_monitor: perintah tidak ditemukan
udisks2_monitor: príkaz nenájdený
udisks2_monitor: příkaz nenalezen
udisks2_monitor: команда не найдена
udisks2_monitor: командата не е открита
udisks2_monitor: 命令找不到
udisks2_monitor: コマンドが見つかりません