AWS: None of the Instances are sending data

I'm trying to set up an Elastic Beanstalk application with Amazon Web Services however I'm receiving a load of errors with the message None of the instances are sending data. I've tried deleting the Elastic Beanstalk Application and the EC2 instance several times with the sample application and trying again but I get the same error.

I also tried uploading a flask application with AWS Elastic Beanstalk command line tools but then I received the error below:

Environment health has transitioned from Pending to Severe. 100.0 % of the requests to the ELB are failing with HTTP 5xx. Insufficient request rate (0.5 requests/min) to determine application health (7 minutes ago). ELB health is failing or not available for all instances. None of the instances are sending data

Why do I get this error and how do I fix it? Thanks.