Hi 
all configuration options besides server_urls are optional. To me, it seems the url might not be valid/reachable.
As you can see in https://www.elastic.co/guide/en/apm/agent/java/current/trouble-shooting.html#trouble-shooting-logging, it's expected to see a log line similar to this one in the logs:
[apm-server-healthcheck] INFO co.elastic.apm.agent.report.ApmServerHealthChecker - Elastic APM server is available: {"build_date":"2018-11-05T07:58:08Z","build_sha":"dffb98a72a262ca22adad0152f0245ea743ea904","version":"7.0.0-alpha1"}
Could you please upload your complete your complete log file? See also https://www.elastic.co/guide/en/apm/agent/java/current/trouble-shooting.html