I tried uploading an app there once, at the final stage they ask to produce some intellectual property rights.
I wrote back to them that the name, icon was all designed by me but nothing happened...
Below is what they wrote to me. I wasn't able to figure out what they want, whether I need to get a lawyer to draft something for me, I don't know so I gave up.
Please provide documentation demonstrating that you have valid intellectual property rights must be either a confirmation letter from the intellectual property rights holder with a formal signature or a copy of the applicable license agreement, like Google console if your application is available on the Google Playstore. We are unable to accept informal claims of rights.
I have multiple apps in Amazon App Store & never received a message like this. Maybe there was something in your app that they erroneously identified as belonging to someone else? If you produced the assets yourself it's a bit hard to provide documentation to prove you own the IP.