It's highly recommended you avoid using the current implementation of Solder unless you already had a mod repository setup before the Platform.
The reason for this being is the current version of Solder is very user error prone. It can't do much to check if mods actually exist in its current state in the master branch.
If you are hell bent on trying Solder, play with the dev branch. It's way less error prone. In fact, I am making a video on it now.