On Android, you cannot buy books through the Kindle or Amazon app. By the terms and conditions of the Google Play store, Google receives 30% of the sales of any digital purchase made within the app. Apparently this doesn't apply to physical goods which is why stores still have apps. This is also why if you may not be able to purchase a subscription in an app or why your subscription would be 30% more if you purchase it through the app.
For Kindle, You have to go into the Amazon web page through your browser in order to buy books. Similarly, for Audible, you can use credits to purchase audiobooks but in order to buy the book with cash you have to use the browser.
I checked and I can see the books within my browser. You can try these lines for light novels volume 14 through 16.