This indicates that there is a problem authenticating the signing key for Chrome may be missing from your system.
This is stopping your update/upgrade from working.
To help diagnose the problem, please can you open a terminal - by pressing Ctrl Alt and T together.
In the terminal window type or paste in this command followed by Enter.
apt-key list
This will show what keys apt can find to check software is signed.
Copy the output and paste it back here.