It seems the stack has something to do with it. Can you delete it and try again?

$ sls remove

If it does not work, try to delete the stack manually in CloudFormation in AWS console.

https://docs.aws.amazon.com/ja_jp/AWSCloudFormation/latest/UserGuide/cfn-console-delete-stack.html

And after that, make sure of deleting S3 bucket as well. It should have something like this, landingpage-dev-serverlessdeploymentbucket-xxxx