I have a CNAME record pointing from the subdomain I bought sub.example.com to my load balancer example.region.elb.amazonaws.com. When I visit https://sub.example.com the website works fine but when I visit the load balancer it's telling me that the certificate is invalid.
I know this has been asked a few times, but the difference is that my setup works on the domain I bought not on the load balancer itself. So how do I make the certificate valid on ACM? Or is it fine to leave it as is, since https works fine on sub.example.com? I'm using cloudflare DNS and cloudflare's origin certificate if that matters.
You can't have SSL certificate for elb.amazonaws.com, because this is not your domain, but it belongs to AWS. To get a valid SSL cert you need to have your own domain. You can buy it using Route53 or any other domain registrar.