how to get back a domain after delete an environment

Please check your Route53 records for this domain, you might have it pointing to old destination

When you delete an Elastic Beanstalk environment and then recreate it, even with the same name, AWS assigns a new environment URL (like mydomain.rxazvzi4mv.ap-southeast-2.elasticbeanstalk.com). This new URL is unique and different from the one assigned to the previously deleted environment.

To associate your custom domain (staging.mydomain.com) with the new Elastic Beanstalk environment, you'll need to update your DNS settings where your domain is managed (e.g., Route 53, GoDaddy, etc.) to point to the new Elastic Beanstalk environment URL.