EC2 instances

You'll notice in the documentation that only ELBs and CloudFront are supported by ACM certificates. You cannot put an ACM certificate directly on an EC2 instance.

While this is a limitation, in practice it's not a big issue. Generally, you wouldn't want to expose your instances to the Internet directly—they should be behind an ELB/ALB for security, performance, and management reasons. If you are serving static assets, CloudFront is going to be much more secure, and performs better at a lower cost.