I was trying to set up RPi 0 2 to broadcast a WiFi AP and use WiFi client mode simultaneously, but couldn't update Raspberry Pi OS.

In order to update the OS, I had to use the fix from this thread on Github discussions.

Here's the fix:

How to Fix NO PUBKEY in Raspberry Pi OS

(I'm using Raspberry Pi Zero 2 W.)

sudo apt-key adv --keyserver keyserver.ubuntu.com --recv-keys 04EE7237B7D453EC

sudo apt-key adv --keyserver keyserver.ubuntu.com --recv-keys 648ACFD622F3D138

 

Then do:

sudo apt update