Prerequisite Checks Failed while agent deployment EM12c

Problem:

Recently when I was trying to deploy management agent in Enterprise Manager  12c using "Add Targets Manually" , I was getting the below error.

Remote Prerequisite Checks Failed.

 

Possible Cause:

• It seems I was trying to install agent on same OMS ( OEM12c R2 (12.1.0.2)) machine.
• An EM agent home already exists on the host.
• 11g and 12c Agents are already installed on the same server where we are trying to install 12c agent.

 Solution:

• This is just a Warning Message, you can ignore and proceed with the new 12c Agent Installation; if the existing Agent is communicating with a different OMS. 
• But as the  existing 12c agent is communicating with the same 12c OMS, so now I'm trying to deinstall the Oracle Management Agent home and retry the agent deployment part.

Prerequisites

Before you deinstall a Management Agent, do the following:

• Shut down the Management Agent by running the following command from its home. If it is already shut down, then skip this step.

             $<AGENT_HOME>/bin/emctl stop agent

•     Wait for the Management Agent to go to the unreachable state in the Cloud Control console. If it is already in the unreachable state, then go to the next step.

•     Delete the Management Agent targets and their monitored targets (from any host where EM CLI is installed):

    emcli delete_target
    -name="example.com:1836"
    -type="oracle_emd"
    -delete_monitored_targets

There are 4 ways to deinstall Oracle Management agent, I will be discussing the one which is most recommended and highly used:

 

1. Using the AgentDeinstall.pl Script
2. Using the Installation Wizard in Graphical Mode
3. Using the Installation Wizard in Silent Mode
4. Using an RPM File

Deinstalling Oracle Management Agents Using the Installation Wizard in Graphical Mode 

          1. Go to the agent home directory and invoke OUI(installer)

                cd $AGENT_HOME/oui/bin
./runInstaller.sh -deinstall ORACLE_HOME=<absolute_path_to_agent_home> [-removeallfiles] 

          2. In the installation wizard, click Installed Products.  

          3. On the Inventory screen, select the plug-in homes under the required Management 

              Agent home, then click Remove. 

          4. On the Inventory screen, select the sbin12cX home, and click Remove. 

          5. On the Inventory screen, select the Management Agent, and click Remove. 

          6. Manually delete the agent base directory. For information on installation base directory.

                     For UNIX platforms: 

             rm -rf <absolute_path_to_agent_base_dir>

After deinstalling the Oracle Management Agent I was able to deploy the agent successfully.

References:

EM 12c: Troubleshooting 12c Management Agent Installation issues (Doc ID 1396675.1)