Issue launching a simple Elastic Beanstalk environment

I'm attempting to get a simple Elastic Beanstalk environment off the ground, and I'm getting an error during environment creation process. I've made very few changes away from the default configuration values.

Some of the settings I have made:

• Platform: Ruby 3.0
• Amazon Linux 2/3.6.4
• Starting with the Rails sample application
• DB engine enabled: postgres, 14.6, db.t3.micro
• I've set an existing EC2 keypair
• I've chosen my organization's default_vpc
• I've added just one subnet/availability zone
• I've not chosen any EC2 security group

The error I'm seeing is:

Stack named 'awseb-e-****-stack' aborted operation. Current state: 'CREATE_FAILED' Reason: The following resource(s) failed to create: [AWSEBRDSDBSubnetGroup].

I've launched two environments this way, with the same result. Since the environment doesn't ever enter the ready state it becomes impossible to try further configuration changes.

I also tried launching an environment with no database to start, and adding one afterward, with a very similar error:

Service:AmazonCloudFormation, Message:Stack named 'awseb-e-****-stack' aborted operation. Current state: 'UPDATE_ROLLBACK_IN_PROGRESS' Reason: The following resource(s) failed to create: [AWSEBRDSDBSecurityGroup, AWSEBRDSDBSubnetGroup].

I can't tell if this is due to something I've misconfigured, or if something is wrong deeper in the stack that I can't influence. Happy to post more details if needed.