unable to get letsencrypt certificate

Hello.

Is HTTP allowed in EC2's security group inbound rules?
Let's Encrypt's HTTP challenge will fail if the web server cannot be reached on port 80.
https://letsencrypt.org/docs/challenge-types/

Please check the following document for examples of security groups used on web servers.
https://docs.aws.amazon.com/AWSEC2/latest/UserGuide/security-group-rules-reference.html

Please check the following document for adding rules to security groups.
https://docs.aws.amazon.com/AWSEC2/latest/UserGuide/working-with-security-groups.html#adding-security-group-rule

In addition to Riku's answer above, have you checked out AWS Certificate Manager? I've found it to be a pretty convenient way to create & manage my certs without needing to install letsencrypt certbot.

You can attach the cert to the load balancer whether through the console, CLI, or infrastructure as code (CloudFormation)