I am having the same issue. I found this, brace yourself, it is not supported

https://repost.aws/questions/QU_f3P9W5KQ7KKc4xVaiHqLQ/how-can-i-use-the-data-manager-api-playground-ui-in-aws-amplify-gen2-for-my-sandbox-environment

The passage of interest is "Unfortunately, the Data Manager UI in the AWS Amplify Console is not available for sandbox environments in AWS Amplify Gen 2. The Data Manager and API Playground features are designed for use with deployed environments, typically associated with Git branches in your repository.Unfortunately, the Data Manager UI in the AWS Amplify Console is not available for sandbox environments in AWS Amplify Gen 2. The Data Manager and API Playground features are designed for use with deployed environments, typically associated with Git branches in your repository."

It does fly in the face of do whatever you want and we do the hard stuff. Amazon have chosen sandbox deploy speed over functionality. I would love to see a local sandbox manager. It would not that could look at local sandboxes and cloud deployments.

Going to add this reply to the other thread