Please find here main repositories needed for softwares used by other Tutorials (Teamviewer, openssh, etc).
Log first as root in terminal by typing “su” and validating with root password.
1) Update the list of repositories for the Debian packages by editing the file "/etc/apt/sources.list".
To edit the file, type in terminal:
pluma /etc/apt/sources.list
The repositories should be:
deb http://deb.debian.org/debian buster main
deb http://deb.debian.org/debian-security/ buster/updates main
deb http://deb.debian.org/debian buster-updates main
Disable all other repositories in the file by putting a # symbol in front of their lines.
Warning: These repositories are for Debian buster.
2) Save the changes before exiting the file.