Thank you, @Sirtel and @Deskisamess. I rebooted several times. It didn’t help.
I thought, ok I’ll try downloading to my Windows laptop. From the device content page, every place to download was greyed out so I couldn’t download it directly to the hard drive. I have a Kindle app on the laptop, so I opened it. But it wouldn’t let me sign in, kept saying my email or password were wrong. However, I kept getting the verification codes by text, so I knew my email and password were correct. So...app outdated perhaps? It came preinstalled on the laptop, purchased about 4 years ago, and I’ve never used it. Actually, these days the laptop seldom even gets powered on and the only reason it was today is so I could deal with this issue. I considered downloading and installing the app from the Amazon website on my laptop, and I got thinking, “This is a whole lot of hassle for a single $1 book.” I walked away for a while, then decided to try something I’ve never done before: connect my Kindle to my laptop by USB. I did that, and then went to the content page in a browser to download the title. The Kindle showed up in the dropdown menu as a download destination and the instructions said to download it to the documents folder in the Kindle.
Problem solved!
