dpkg-unhold: 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/dpkg-unhold - from package: dlocate

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

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