moc-code-filename: layout/theme.liquid
moc-code-filename: templates/page.sub-collections.liquid

You do not have a link list with a handle set to generators, or you do and it's empty. Go ahead and create a link list called Generators - Machinery on your Navigation page and populate that link list with links that point to collections, to see collections listed here. Make sure the link list also has a handle set to generators.