You are using Enhanced Health Monitoring.
With enhanced health monitoring an agent installed on your EC2 instance monitors vital system and application level health metrics and sends them directly to Elastic Beanstalk.
When you see an error message like "None of the instances are sending data", it means either the agent on the instance has crashed or it is unable to post data to Elastic Beanstalk due to networking error or some other error.
For debugging this, I would recommend downloading "Full logs" from the AWS console. You can follow the instructions for getting logs in the section "Downloading Bundle Logs from Elastic Beanstalk Console" here.
If you are unable to download logs using the console for any reason you can also ssh to the instance and look at the logs in /var/log.
You will find logs for the health agent in /var/log/healthd/daemon.log.
Additional logs useful for this situation are /var/log/cfn-init.log, /var/log/eb-cfn-init.log and /var/log/eb-activity.log. Can you look at the logs and give more details of the errors you see?
This should hopefully give you more details regarding the error "None of the instances are sending data".
Regarding other health "causes" you are seeing:
Environment health has transitioned from Pending to Severe - This is because initially your environment health status is Pending. If the instances do not go healthy within grace period health status transitions to Severe. In your case since none of the instances is healthy / sending data, the health transitioned 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).
Elastic Beanstalk monitors other resources in addition to your EC2 instances when using enhanced health monitoring. For example, it monitors cloudwatch metrics for your ELB. This error means that all requests sent to your environment CNAME/load balancer are failing with HTTP 5xx errors. At the same time the request rate is very low only 0.5 requests per minute, so this indicates that even though all requests are failing, the request rate is pretty low. "7 minutes ago" means that information about ELB metrics is slightly old. Because Elastic Beanstalk monitors cloudwatch metrics every few minutes, so the data can be slightly stale. This is as opposed to health data we get directly from the EC2 instances which is "near real time". In your case since the instances are not sending data the only available source for health is ELB metrics which is delayed by about 7 minutes.
ELB health is failing or not available for all instances
Elastic Beanstalk is looking at the health of your ELB, i.e. it is checking how many instances are in service behind ELB. In your case either all instances behind ELB are out of service or the health is not available for some other reason. You should double check that your service role is correctly configured. You can read how to configure service role correctly here or in the documentation. It is possible that your application failed to start.
In your case I would suggest focusing on the first error "None of the instances are sending data". For this you need to look at the logs as outlined above. Let me know what you see in the logs. The agent is started fairly early in the bootstrap process on the instance. So if you see an error like "None of the instances are sending data", it is very likely that bootstrap failed or the agent failed to start for some reason. The logs should tell you more.
Also make sure you are using an instance profile with your environment. Instance profile allows the health agent running on your EC2 instance to authenticate with Elastic Beanstalk. If instance profile is not associated with your environment then the agent will not be able to send data to Elastic Beanstalk. Read more about Instance Profiles with Elastic Beanstalk here.
Update
One common reason for the health cause "None of the instances are sending data" can be that your instance is in a VPC and your VPC does not allow NTP access. Typical indicator of this problem is the following message in /var/log/messages: ntpdate: Synchronizing with time server: [FAILED]. When this happens the clock on your EC2 instance can get out of sync and the data is considered invalid. You should also see a health cause on the instances on the health page on the AWS web console that tells you that instance clock is out-of-sync. The fix is to make sure that your VPC allows access to NTP.
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