The world builder has changed the name of the main branch from `master` to `main`.
If you are new to the world builder, you don't need to worry about this and you can just clone the repository as usual. If you already have a local cloned repository, github should show instructions on how to update your repository, or you can take a look here: https://github.com/GeodynamicWorldBuilder/WorldBuilder/issues/350#issuecomment-954315494 for instructions and more information.