All of the below should be done as root / sudo su . Caution is advised when operating as root or sudo su. Accidental and careless usage might be dangerous to system stability. Proceed with your own responsibility.

On a latest release of PostX Gnu/Linux contrib and non-free sources entries were accidentally left out. This will be fixed on later upcoming releases. For now you can add the following entries to sources list files:

deb http://deb.debian.org/debian bullseye contrib non-free
deb-src http://deb.debian.org/debian bullseye contrib non-free

deb http://deb.debian.org/debian bullseye-updates contrib non-free
deb-src http://deb.debian.org/debian bullseye-updates contrib non-free

deb http://security.debian.org/debian-security/ bullseye-security contrib non-free
deb-src http://security.debian.org/debian-security/ bullseye-security contrib non-free

Adding entries can be done manually by adding them into /etc/apt/sources.list. Alternatively, they can be added to file of your choosing under /etc/apt/sources.list.d directory structure.

For example you can make a file called patch.sh with the sources list content, as seen above, and make it executable with chmod +x patch.sh .

Finally, you can execute the patch.sh as follows: cat patch.sh > /etc/apt/sources.list.d/extra.list .

This will create a new file called extra.list under /etc/apt/sources.list.d folder. To apply changes to system, you still need to execute apt-get update. After this you are all done and new sources should be ready for usage.