Can't install Jitsi on Debian 11

Not able to install Jitsi using this guide:

at step “sudo apt-get -y update”
“Err:7 https://download.jitsi.org stable/ InRelease
The following signatures couldn’t be verified because the public key is not available: NO_PUBKEY B4D2D216F1FD7806
Reading package lists… Done
W: GPG error: https://download.jitsi.org stable/ InRelease: The following signatures couldn’t be verified because the public key is not available: NO_PUB KEY B4D2D216F1FD7806
E: The repository ‘https://download.jitsi.org stable/ InRelease’ is not signed.
N: Updating from such a repository can’t be done securely, and is therefore disabled by default.
N: See apt-secure(8) manpage for repository creation and user configuration details.”