Pessoal, instalei o Browser Vivaldi e após rodar um update, as seguintes mensagens aparecem no terminal:
W: GPG error: Index of /stable/deb/ stable Release: The following signatures couldn’t be verified because the public key is not available: NO_PUBKEY 9658E8044A3AA3D6
E: The repository ‘Index of /stable/deb/ stable Release’ 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.
Tentei adicionar repositórios do vivaldi mas nada aconteceu… poderiam me dar um caminho para resolver este problema?