OEM 13c Discovering WebLogic Domain failed to save Node Manger target with error 'This target requires a local Management Agent'

When discovering or refreshing a WebLogic Domain or Fusion Middleware Farm in Enterprise Manager (EM) 13.4 Cloud Control, the Node Manager target is not saved. The error is shown in EM:

Failed to save NM_xxx_x(Oracle WebLogic Node Manager) on host <IP/host name>. This target requires a local Management Agent, but a local Management Agent was not found.  In order to add this target, you need to install a Management Agent on the same host as the target and then perform a "Refresh WebLogic Domain" operation.

The agent has been installed on the host. The errors happened because of difference between Listen Address of Node Manager configuration and host name of EM Agent URL. As a solution, the Listen Address Node Manager should be changed to host name of EM Agent URL.

Oracle explains it as incorrect configuration of Oracle WebLogic Node Manager. Therefore, it could happen on all release of EM 13c. However, I can only reproduce the problem in EM 13.1 and 13.4 when Listen Address of WebLogic Node Manager is configured with IP address instead of host name which is used by EM Agent URL and there is no problem with EM 13.2. Anyway, having both configuration use same host name is not bad idea.

Find out host name of EM agent URL with command <AGENT_HOME>/bin/emctl status agent

$ /u01/app/oracle/em13.4/agent/agent_13.4.0.0.0/bin/emctl status agent
Oracle Enterprise Manager Cloud Control 13c Release 4
Copyright (c) 1996, 2020 Oracle Corporation.  All rights reserved.
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Agent Version          : 13.4.0.0.0
OMS Version            : 13.4.0.0.0
Protocol Version       : 12.1.0.1.0
Agent Home             : /u01/app/oracle/em13.4/agent/agent_inst
Agent Log Directory    : /u01/app/oracle/em13.4/agent/agent_inst/sysman/log
Agent Binaries         : /u01/app/oracle/em13.4/agent/agent_13.4.0.0.0
Core JAR Location      : /u01/app/oracle/em13.4/agent/agent_13.4.0.0.0/jlib
Agent Process ID       : 76282
Parent Process ID      : 76240
Agent URL              : https://host01.lab.dbaplus.ca:3872/emd/main/
Local Agent URL in NAT : https://host01.lab.dbaplus.ca:3872/emd/main/
Repository URL         : https://oms.lab.dbaplus.ca:4903/empbs/upload
Started at             : 2021-04-07 17:53:56
Started by user        : oracle
Operating System       : Linux version 4.1.12-124.46.4.1.el7uek.x86_64 (amd64)
...

---------------------------------------------------------------
Agent is Running and Ready

Change Listen Address of Node Manager to the host name of EM Agent URL in the WebLogic Admin Console as following,

1. Go to Node Manger configuraiton page

   Environment > Machines > [Machine Name] > Configuration > Node Manager

2. Click 'Lock & Edit' to enable edit mode

3. Set the value of "Listen Address" property to the host name given by previous command 'emctl status agent'

4. Click 'Save', then click 'Activate Changes'

Refresh or rediscover the domain, the Node Manger will be discovered successfully.