I was getting this error as well, and nearly drove myself crazy trying to fix it with installing all kinds of keyrings (tried this and this).

Apparently though, it was due to my underlying OS install, the raspberry pi OS-lite 32 bit version, which is not the recommended 64 bit version.

Check out this thread for more information: https://forum.openmediavault.org/index.php?thread/51223-apt-update-public-key-errors/