This error is due to an outdated debian repository. The backports repository moved from debian to debian archive. You need to change the URL to the repository.
Solution for Debian buster:
Find the old source with
grep -r "stretch-backports" /etc/apt/
Sample output:
/etc/apt/sources.list.d/backports.list:deb http://ftp.debian.org/debian stretch-backports main
The first part is the file where the source is configured:
/etc/apt/sources.list.d/backports.list
The second part is your configured repository:
deb http://ftp.debian.org/debian stretch-backports main
Now in the file /etc/apt/sources.list.d/backports.list you need to change the repository URL from http://ftp.debian.org/debian to archive.debian.org
You can do that with a text editor like nano:
nano /etc/apt/sources.list.d/backports.list
Or automated replacing with
grep -rl "deb http://ftp.debian.org/debian stretch-backports main" /etc/apt/ | xargs sed -i 's/ftp.debian.org/archive.debian.org/g'
(Note: Pay attention to the repository URL. The configured server may be deb.debian.org or ftp.debian.org, so automated replacing may go wrong.)
After this you can apt update && apt upgrade again.