During apt-get update you can get error:

Err:3 http://apt.radxa.com/bullseye-stable bullseye InRelease    
  The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 9B98116C9AA302C7

(...)

Reading package lists... Done                                                                                            
W: GPG error: http://apt.radxa.com/bullseye-stable bullseye InRelease: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 9B98116C9AA302C7
E: The repository 'http://apt.radxa.com/bullseye-stable bullseye 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.

You have to just type:

export DISTRO=bullseye-stable
curl -o - apt.radxa.com/$DISTRO/public.key | sudo apt-key add -

Where bullseye-stable is your distro from above error. Now you can do apt-get update again.

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