It might be that your ISP is using a transparent proxy which contains incorrect data.
If you’re familiar with the command line, you can edit file
/etc/apt/sources.list to this:
deb http://ftp.debian.org/debian buster main contrib non-free
deb http://ftp.debian.org/debian/ buster-updates main contrib non-free
deb http://security.debian.org/ buster/updates main contrib non-free
# deb http://apt.osmc.tv buster main
deb https://ftp.fau.de/osmc/osmc/apt/ buster main
(I’ve commented out the last line and added a new line using https.)
sudo apt-get update
sudo apt-get dist-upgrade
This should at least bypass any dodgy ISP proxy, should one exist.