Can't create Amazon issued certificate of my BeanStalk domain

It sounds like you're creating a certificate in Amazon Certificate Manager (ACM). It would be helpful to know whether the domain for your certificate is managed by Route 53 or an external service. Either way, you need to verify that you own the domain or have admin access to it, just it is easier to do that when using Route 53. There are a couple ways to verify the domain, creating CNAME records in your domain's DNS entry is the best in my opinion because it allows for automated cert renewal. ACM will tell you the records you need to create, and if your domain is hosted by Route 53 it will give you an option to automatically create them. Take a look at the following for reference: https://docs.aws.amazon.com/acm/latest/userguide/domain-ownership-validation.html