krb5-sync-backend: 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/sbin/krb5-sync-backend - from package: krb5-sync-tools

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

Error in other languages:
krb5-sync-backend: bevel nie gevind nie
krb5-sync-backend: comandă negăsită
krb5-sync-backend: comando não encontrado
krb5-sync-backend: commande introuvable
krb5-sync-backend: command not found
krb5-sync-backend: käsku ei ole
krb5-sync-backend: không tìm thấy lệnh
krb5-sync-backend: komanda nerasta
krb5-sync-backend: Kommando nicht gefunden.
krb5-sync-backend: kommandot finns inte
krb5-sync-backend: komut yok
krb5-sync-backend: nie znaleziono polecenia
krb5-sync-backend: níor aimsíodh an t-ordú
krb5-sync-backend: no se encontró la orden
krb5-sync-backend: no s'ha trobat l'ordre
krb5-sync-backend: opdracht niet gevonden
krb5-sync-backend: parancs nem található
krb5-sync-backend: perintah tidak ditemukan
krb5-sync-backend: príkaz nenájdený
krb5-sync-backend: příkaz nenalezen
krb5-sync-backend: команда не найдена
krb5-sync-backend: командата не е открита
krb5-sync-backend: 命令找不到
krb5-sync-backend: コマンドが見つかりません