clamav-unofficial-sigs: 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

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/clamav-unofficial-sigs - from package: clamav-unofficial-sigs

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