Please fill out the fields below so we can help you better. Note: you must provide your domain name to get help. Domain names for issued certificates are all made public in Certificate Transparency logs (e.g. https://crt.sh/?q=example.com), so withholding your domain name here does not increase secrecy, but only makes it harder for us to provide help.
Okay. So your domain points at your ELBs. That 503 comes from an ELB when there is no backend available/attached to it:
$ curl -i letsencript.freshpo.com
HTTP/1.1 503 Service Unavailable: Back-end server is at capacity
Content-Length: 0
Connection: keep-alive
Can you confirm that you can spin up a web server on port 80 on the EC2 instance, and that you can access it via the ELB? You won’t be able to issue a certificate via Certbot unless the ELB actually routes traffic to your instance.
Once that works, you should be able to run certbot in standalone mode.
Also, if you’re not aware, you can get free automatically renewing certificates that integrate with Amazon ELB via Amazon ACM. It may be a superior solution to Let’s Encrypt in this instance.