When apt-get update shows the following error with only debian repositories:

W: There is no public key available for the following key IDs:

The simple fix is to install the debian-archive-keyring package and repeat the update process

apt-get install debian-archive-keyring
apt-get update

For google chrome repository and error:

W: There is no public key available for the following key IDs:
1397BC53640DB551

The fix is to:

wget -q -O - https://dl.google.com/linux/linux_signing_key.pub | sudo apt-key add -

For other NO_PUBKEY errors, make a note of the ID, then use

sudo apt-key adv --keyserver keyserver.ubuntu.com --recv-keys KEY

In case of dpkg dependency errors of a package that isn't configured yet, trigger configuration manually.
Example error:

 dpkg: dependency problems prevent processing triggers for libglib2.0-0:i386:
 libglib2.0-0:i386 depends on libc6 (>= 2.28); however:
  Package libc6:i386 is not configured yet.

sudo dpkg --configure -a 

Apt Error: “Warning: apt-key is deprecated. Manage keyring files in trusted.gpg.d instead (see apt-key(8)).”

Find the repository causing the error. Note that the apt-key output does not contain the url but descriptions and it might be difficult to find the relevant entry.

sudo apt-key list

Once the entry is found, use the last 2 blocks of 4 characters of the key id to export it and then create a separate gpg file fir it:

sudo apt-key export 12345678 | sudo gpg --dearmour -o /etc/apt/trusted.gpg.d/name-of-file.gpg

Finally, the old key id can be removed from the trusted.gpg file:

sudo apt-key --keyring /etc/apt/trusted.gpg del 12345678