The apt packaging system has a set of trusted keys that determine whether a package can be authenticated and therefore trusted to be installed on the system. Sometimes the system does not have all the keys it needs and runs into this issue. Fortunately, there is a quick fix. Each key that is listed as missing needs to be added to the apt key manager so that it can authenticate the packages.
Please try:
sudo apt-key adv --keyserver keyserver.ubuntu.com --recv-keys E5267A6C && sudo apt update
