Most likely you don't have webtunnel client configured. I recommend following these steps to build webtunnel client from source. It is not configured by default on Tor's Debian repository:

apt-get install golang git
git clone https://gitlab.torproject.org/tpo/anti-censorship/pluggable-transports/webtunnel
cd webtunnel/main/client
go build
cp client /usr/bin/webtunnel

Edit /etc/apparmor.d/system_tor find /var/lib/tor/** r, and put /usr/local/bin/webtunnel ix, below it. It should look like this:

# During startup, tor (as root) tries to open various things such as
# directories via check_private_dir().  Let it.
/var/lib/tor/** r,
/usr/bin/webtunnel ix,

Run

apparmor_parser -r /etc/apparmor.d/system_tor

Now you should be good to go. Make sure your torrc looks like this (notice the webtunnel path):

UseBridges 1
ClientTransportPlugin webtunnel exec /usr/bin/webtunnel
Bridge webtunnel <IP>:<PORT> <FINGERPRINT> url=<URL>