In /etc/entropy/repositories.conf you find the general configuration that count for all repositories. The different repositories are inherited from a subdirectory: /etc/entropy/repositories.conf.d
In this subdirectory you should find some predefined examples for the different repositories.
However, in some cases, some how those example files are missing, so you'll have to create them manually.
If this is the case, then follow the below instructions to create them, and eventually enable those you wish to enable.
Open your favourite terminal and become root.
Navigate to the subdirectory:
# cd /etc/entropy/repositories.conf.d