Hi Douglas,
I am one of the course authors. I'm not sure what is wrong. However, when I first go in to the Living Atlas tab, I see there are 8212 items, while you say there are over 3400. Before I click anything, the Environmnet category shows 1887 items available, and Earth observations shows 291 items. Is that what you see? I'm wondering if there is an unintended filter turned on or a keyword in the search box, which is limiting your results?
Here are some other suggestions to try. After each one, see if anything shows up when you filter.
1. Make sure no stray filters are turned on. Click "clear" to clear the filters, and click the X to clear the search box.
2. Try clearing your browser cache and then reloading the page.
3. Try using a private or incognito browser window.
4. Try using a different browser.
5. Verify that your admin has not limited the ability for users to view outside content. (For example, do you get any results at all if you do a search of ArcGIS Online and toggle off "Only search in <your organization name>"?
Laura Bowden