Jenny, Amazon has stopped selling it's Kindle ebooks through Apple because they charge too much, so you were correctly told you cannot buy it from your iPad app.
You can buy it on your iPad another way, though, explained here:
https://www.businessinsider.com/guides/tech/how-to-buy-kindle-books-on-ipad?r=US&IR=T
I am confused as to why the book you bought didn't show up in your iPad Kindle app. If you were logged in to your Amazon account in the Kindle app the book should have appeared in the app's library