Healthcheck of ALB registered as a target

First of all check your security groups and NACLs that they are allowing traffic. verify that path / is appropriate for the application running behind the ALB Ensure that the target group's protocol and port settings match the protocol and port on which the application is running. Ensure that the target instances behind the ALB are in healthy state. Check the ALB's listeners to make sure that they are correctly configured to route traffic to the target group. verify that the ALB's SSL/TLS settings match the protocol and settings of the target instances. If necessary, enable access logs for the ALB to get more insights into the traffic flow and any potential issues.

for a more detailed methodology see https://repost.aws/knowledge-center/elb-fix-failing-health-checks-alb