So you’ve not done anything to tell NameSilo about your EC2 server at Amazon?
The way DNS works is that you tell the DNS server about the resources you’re trying to publish under that domain name. You need to configure an ‘a’ record at NameSilo for your Discourse instance, linked to the IP at Amazon.
Your screenshot shows discourse.example.com - I’m assuming that’s not the domain you’ve trying to work with. It might help if you give us more information about the specific configuration so that we can advise on which records to create.
NameSilo and/or Amazon aren’t psychic, they have no knowledge of one another, it’s up to you to create the necessary DNS entries so that one refers to the other.
Once DNS is correctly configured you will need to wait for the records to propagate, then rebuild your app (./launcher rebuild app) so that Let’s Encrypt can enroll the certificate.