appstream-validate: 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/appstream-validate - from package: appstream-index

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

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