Here a Graphical tutorial to set main repositories on Debian 10 Buster with Synaptic Package Manager
1) Go to System > Administration > Synaptic Package Manager
Root password will be prompted
2) In Synaptic, go to Settings > Repositories
3) Be sure that you have those three configured :
a) Binary (deb)
URL : http://deb.debian.org/debian
Distribution : buster
Section(s) : main
b) Binary (deb)
URL : http://security.debian.org/debian-security/
Distribution : buster/updates
Section(s) : main
c) Binary (deb)
URL : http://deb.debian.org/debian
Distribution : buster-updates
Section(s) : main
You can use Up and Down to set order, and uncheck other repositories.
4) Clicking on OK will validate your settings. If there is a warning about "Repositories changed", click on Reload :
5) Quit Synaptic.