What is causing my attempt to enable SSH for AWS-E

I'm trying to enable SSH for my AWS Elastic Beanstalk application and have run eb ssh --setup (as a user with what seem to be suitable privileges, ElasticBeanstalkFullAccess; using AWS CLI 3.x); but my attempt fails with the following (GUIDs changed to protect the innocent):

INFO: Environment update is starting.
INFO: Updating environment sitetest-develop-env's configuration settings.
INFO: Created Auto Scaling launch configuration named: awseb-e-notrea1nUm-stack-AWSEBAutoScalingLaunchConfiguration-MAdUpa2bCrCx
ERROR: Updating Auto Scaling group failed Reason: Template error: DBInstance bxzumnil42x11w doesn't exist
ERROR: Service:AmazonCloudFormation, Message:Stack named 'awseb-e-notrea1nUm-stack' aborted operation. Current state: 'UPDATE_ROLLBACK_IN_PROGRESS'  Reason: The following resource(s) failed to update: [AWSEBAutoScalingGroup]. 
ERROR: Failed to deploy configuration.
INFO: Created Auto Scaling launch configuration named: awseb-e-myjrm7xr9n-stack-AWSEBAutoScalingLaunchConfiguration-5uKixPQCM71K
INFO: Deleted Auto Scaling launch configuration named: awseb-e-notrea1nUm-stack-AWSEBAutoScalingLaunchConfiguration-MAdUpa2bCrCx
INFO: The environment was reverted to the previous configuration setting.

What is causing this to happen? Is there something I need to do in the AWS Console to prevent this error?

the relevant error message i see here is DBInstance bxzumnil42x11w doesn't exist. You have probably opted into letting Elastic Beanstalk create an RDS server as part of the environment creation process. Now it seems the db is no longer there. Did you kill it manually?
In any case, I would recommend NOT to let EB manage your RDS. it's best practice to create one yourself and manually assign the following environment variables: RDS_HOSTNAME, RDS_PORT, RDS_DB_NAME, RDS_USERNAME, RDS_PASSWORD.
At this point I would recommend terminating this env and creating a new one, only this time don't check the checkbox named Create an RDS DB Instance with this environment.