dpkg-sig: command not found

Have You tried accessing the binary with the full path?

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/dpkg-sig - from package: dpkg-sig

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