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

I resolved running:

sudo apt-key adv --keyserver keyserver.ubuntu.com --recv-keys 1397BC53640DB551

The command above download the missing key using the hexadecimal numbers given in the error (1397BC53640DB551) with apt-key. After running apt-key to download the missing PUBKEY, any packages from that repositories will be considered trusted.

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It seems to be a known problem in chromium that is being worked on (check here). They are working to remove SHA1 encryption (link).

Another possible workarounds that have been posted on reddit

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

Looking at this, seems that Google now has two keys, and the old one will probably be discontinued along with SHA1.

Correct command line is:

sudo apt-key adv --keyserver keyserver.ubuntu.com --recv-keys 1397BC53640DB551