You've customized your site and suddenly you have no galleries listed on your folder pages. Don't fret!
To get your galleries on the page, open the Customizer and click on "All Folders" in the list at the top of the sidebar. This allows you to globally set things you want to appear anytime someone navigates to a folder on your site.
In the Add Content Blocks section, expand the Galleries option so you can see the Galleries content block.
Now drag the Galleries content block option onto your Folders page and drop it where you want it to appear.
That's it! Click Done to commit your changes and Publish Now to make them live on your site. You should now be able to see all the available galleries.
If you're customizing on a page that doesn't contain any galleries, then no galleries will show. A "no galleries found" message will only appear in the Customizer, not on your live, published site.