Target group - Health checks failed with these codes: [500]

There isn't enough information in your question to provide detailed answers. If you see HTTP 500 errors for the health checks, your instances return that response. I suggest looking at your instances' logs to determine the cause of the HTTP 500 response. Additionally, you could try running your container locally and making a request to the same path as the health check and see if the service behaves the same, returning an HTTP 500 response, or different.