Here’s how to open the book on your Apple phone or Tablet in the Kindle App:
  1. Make sure your device is turned on and connected to wi-fi or mobile internet.

  2. On your mobile phone or tablet, navigate to this page (if you’re not already on your mobile phone or tablet right now).

  3. If you’re using your mobile phone or tablet right now, just click this link to download the ebook. 

  4. If asked “Are you sure you would like to download this file?” tap “Yes”.

  5. The file will download to your device and will be available inside your Kindle App in your books library.

Manual troubleshoot: 

Sign in to your Amazon account if you are already registered. Connect your Android device to your computer through USB cable. After connecting it, you will see your Android device as a folder or drive. You can drag and drop the eBook (mobi) onto the Kindle folder on computer.
Source: https://ostoday.org/android/where-do-i-put-mobi-files-on-my-android.html