Now... this is odd.
Focal Fossa is still supported.
And... Another thread was just opened on a similar issue:
At first, I was thinking that the user added a PicoNets repository.
But after seeing your thread, I did some checking.
The Regional Server for India is using Piconets Webworks to host their packages.
Please switch to the Main Server using Software & Updates
Under Download From: Select Main Server.
Close Software & Updates
and allow it to refresh.
Next, in terminal, run sudo apt update
and ensure that all repositories return a good status.
EDIT: I just contacted the piconets mirror to report the outage, in case they are unaware.
You can remove the duplicate volian sources file in /etc/apt/sources.list.d/
directory to fix this.