Do you mean you made a copy of your iTunes Library to a USB stick?
If yes, good.
What Kind of computer are you using Mac or PC?
ITunes 12.6.3 and/or 12.6.4 requires a Mac running OS X 10.10 Yosemite or greater.
Not sure of the system requirements for the Windows PC.
The problem with iTunes is that with every new version release (12.5, 12.6, 12.7, etc iTunes rebuilds the iTunes Library folder for a particular version of iTunes to work.
If you made a copy of your current iTunes library, what I would do is simply move the iTunes 12.7 version out of the Applications folder and say move it, temporarily, into your user (Home folder under your user name).
Then go back to my post reply where I have linked the location for iTunes 12.6.3 (I think that link is for iTunes 12.6.4, now, but that’s fine) download and install the iTunes 12.6.3/4 version onto your computer.
When you first launch iTunes 12.6.3/4 hold down the opt key (alt key on Windows to create a new iTunes library using this older version.
In iTunes 12.6.3/4, log into the iOS App Store using your Apple ID and password and then download/install the latest Kindle app version. Once this is downloaded to iTunes on your computer,
Go back to your iPad and try to re-download the current Kindle app from the iOS App Store on to your iPad and see if the pop-up message appears asking if you want to install an older, compatible version to your device.
If this message appears, tap the large OK button and it should download/install an older version of the Kindle app onto your iPad.
Once you have achieved this, you have a few options you can try.
If all you have is an older iPad, you can keep using this version of iTunes, if you like.
Try putting back your original iTunes Library from the USB stick ORif you still left the original iTunes Library on your computer, you can try launching iTunes 12.6.3/4 while holding down the opt/alt key and choose your original library and see if iTunes 12.6.3/4 will accept it and if it works.
If not, you can move out iTunes 12.6.3/4 out of your Applications folder, put that some where else for the time being (you may still need this to download other apps through iTunes) and put back the iTunes 12.7 app back into your Applications folder, launch it with the opt/alt key held down and chooss to link it to your original iTunes Library if it is still on your computer.
If you removed your original iTunes Library from you computer, AFTER you had copied it to your USB drive, you will have to copy the iTunes Library back to your computer, first, before attempting the above options.
Good Luck to You!