Yes, just about right question that @Ass113 needs to ask this Linux distro:
apt list pureos-archive-keyring
And if pureos-archive-keyring package not [installed] either:
sudo apt update
sudo apt reinstall pureos-archive-keyring
or
sudo apt install pureos-archive-keyring
If above for some reason doesn’t work (as not retested from my side), please proceed with downloading related package and:
cd Downloads −− or any other related folder
sudo dpkg --install pureos-archive-keyring_2021.11.0_all.deb
sudo apt update
sudo apt upgrade
EDIT:
Official “Debian 9+, Ubuntu 16.04+ and Mint 18+” chapter instructions should solve Brave key issue. And if/how to enable ppa:nilarimogard/webupd8 is another challenge of yours (if you really want to keep this 3rd party repository, please see under technical details for further explanation … on how to enable hirsute main perhaps?).