amazonaws.com happens to be on the blacklist Let’s Encrypt uses for high-risk domain names (i.e. phishing targets, etc.).
There’s nothing you can do about this on your end, other than use your own domain name. If you’re looking for a free option, check out http://www.dot.tk/. CloudFlare provides free DNS servers.
(I believe EC2 instances are kind of ephemeral, so I’m not sure if you can rely on the hostname being the same forever - so this is probably a good idea for other reasons as well. Admittedly, I’m not too familiar with AWS.)