appdata-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/appdata-validate - from package: appdata-tools

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

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