v4l2loopback-ctl: 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/bin/v4l2loopback-ctl - from package: v4l2loopback-utils

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

Error in other languages:
v4l2loopback-ctl: bevel nie gevind nie
v4l2loopback-ctl: comandă negăsită
v4l2loopback-ctl: comando não encontrado
v4l2loopback-ctl: commande introuvable
v4l2loopback-ctl: command not found
v4l2loopback-ctl: käsku ei ole
v4l2loopback-ctl: không tìm thấy lệnh
v4l2loopback-ctl: komanda nerasta
v4l2loopback-ctl: Kommando nicht gefunden.
v4l2loopback-ctl: kommandot finns inte
v4l2loopback-ctl: komut yok
v4l2loopback-ctl: nie znaleziono polecenia
v4l2loopback-ctl: níor aimsíodh an t-ordú
v4l2loopback-ctl: no se encontró la orden
v4l2loopback-ctl: no s'ha trobat l'ordre
v4l2loopback-ctl: opdracht niet gevonden
v4l2loopback-ctl: parancs nem található
v4l2loopback-ctl: perintah tidak ditemukan
v4l2loopback-ctl: príkaz nenájdený
v4l2loopback-ctl: příkaz nenalezen
v4l2loopback-ctl: команда не найдена
v4l2loopback-ctl: командата не е открита
v4l2loopback-ctl: 命令找不到
v4l2loopback-ctl: コマンドが見つかりません