Target group health check, unhealthy

Although you have not provided enough information, I can provide you the link that you can follow through.

The below link seems to provide an end-to-end instruction for your case.

https://www.bogotobogo.com/DevOps/AWS/aws-Autoscaling-Group-Network-Load-Balancer.php

Also, you have to make sure that your EC2 instance is actually in a serving state and has its security group open for Port 80. For example, Apache or Nginx is running and listening to Port 80 (it's listening to Port 80 by default)