Could it have something to do with the following link?

https://docs.aws.amazon.com/elasticbeanstalk/latest/dg/environments-cfg-autoscaling-launch-templates.html

AWS has disabled creating new Launch Configurations and only allows new Launch Templates. But it looks like they haven't fully updated Beanstalk to account for that. According to the link, when creating an environment you need to do one of the following to get Beanstalk to use templates:

• RootVolumeType option set to gp3. You can set this option with the console or the namespace.
• BlockDeviceMappings option contains gp3. You can set this option with the console or the namespace.
• DisableIMDSv1 option set to true. We recommend that you set this option using the namespace.
• EnableSpot option set to true. For more information, see Spot instance support and Configuration Auto Scaling group configuration.