Newcomer Problem: Synaptic

[das <das@randomink.org>]

Hello Friends:

I am using Debian for four or five days only and obviously I did
something stupide while installing and trying to learn the system.

When I am using Synaptic can clicking 'update', it is giving me this
error message
<<
GPG error: http://ppa.launchpad.net/libreoffice/ppa/ubuntu focal
InRelease: The following signatures couldn't be verified because the
public key is not available: NO_PUBKEY 83FBA1751378B444The repository
'http://ppa.launchpad.net/libreoffice/ppa/ubuntu focal InRelease' is
not signed.
>>

How to solve it?

Thanks in Advance

das
Kolkata, WB, India
http://ddts.randomink.org/