I made an “about” page (“Association”) the default start page. When I did that, the tab for the “Blog” page (with posts) continues to appear, but the tab doesn’t work. The only way to view the blog posts is via the archive.
I would like to keep Association as the default start page, but I also need the Blog tab to work.
The tab does work. “Blog” (in other themes “Home” or “Front”) is meant to take you back to your FRONT, which is usually but not necessarily the main posts page. Now that you’ve set a static front, you need to create a BLANK page, name it as you see fit, and set it as your POSTS page (in Settings>Reading).
Also, since “Blog” and “Association” now lead to the same page, there’s no point in having both tabs in your header. You can remove the second by assigning that page to another “parent” page.