Stuck at "Target registration is in progress" in Target group of Load balancer as part of ECS deployment

Hello.

Does the status remain the same even after a few minutes?
In that case, there may be a problem with the task definition or service settings.
I think you need to troubleshoot by checking the task logs and ECS service event messages.
https://docs.aws.amazon.com/AmazonECS/latest/developerguide/using_awslogs.html
https://docs.aws.amazon.com/AmazonECS/latest/developerguide/service-event-messages.html