Just use java -jar atlassian-bamboo-agent-installer-6.7.2.jar https://<server>/agentServer/ and find the issue.
INFO | jvm 1 | 2019/01/17 12:34:34 | 2019-01-17 12:34:34,862 ERROR [AgentRunnerThread] [AbstractFileCapabilityDefaultsHelper] Cannot create capability for C:\Program Files\dotnet\dotnet.exe
INFO | jvm 1 | 2019/01/17 12:34:35 | 2019-01-17 12:34:34,979 ERROR [AgentRunnerThread] [AbstractFileCapabilityDefaultsHelper] Cannot create capability for C:\Program Files\dotnet\dotnet.exe
INFO | jvm 1 | 2019/01/17 12:34:36 | 2019-01-17 12:34:36,016 INFO [AgentRunnerThread] [AgentRegistrationBean] Registering agent on the server,
INFO | jvm 1 | 2019/01/17 12:37:02 | 2019-01-17 12:37:02,508 WARN [ActiveMQ Transport: tcp://serverhost name/serverip:54663@50485] [FailoverTransport] Transport (tcp://serverhost name:54663?wireFormat.maxInactivityDuration=300000) failed , attempting to automatically reconnect: java.net.SocketException: Connection reset