apt-venv: 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/apt-venv - from package: apt-venv

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