readnotes: 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/lib/xen-4.4/bin/readnotes - from package: xen-utils-4.4

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

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