Problem:
At the end of an OEM 13c agent manual installation on one of the servers I got this message:

Finished post install
Plugin txt:
Inside if , is empty
/u01/oracle/agent13c/plugins.txt not found. The Management Agent installation failed. The plug-in oracle.sysman.oh may not be present in the Management Agent software. Ensure that the Management Agent software has the oracle.sysman.oh monitoring and discovery plug-in. 

Reason:
When I copied the agent binary from the OMS server to target server I copied the one located under:
/u01/oracle/13cmiddleware/sysman/agent/13.4.0.0.0_AgentCore_226.zip

This agent was outdated because there were minor changes happened on the OMS since OMS was initially installed.

Fix:
You have to re-generate a new agent from OMS and use it on the target server to install the agent:

On the OMS server: Generate a new agent:

Create a temporary directory that will hold the new generated agent:
# mkdir /tmp/emcli

Login to OMS using emcli with sysman:
# cd /u01/oracle/13cmiddleware/bin
# ./emcli login -username=sysman -password=xxxx
Login successful
# ./emcli sync
Synchronized successfully

Get the platform details:
# ./emcli get_supported_platforms
———————————————–
Version = 13.4.0.0.0
Platform = Linux x86-64
———————————————–
Platforms list displayed successfully.

Generate the agent providing the temp location along with the platform and version you got in the  previous command:

# ./emcli get_agentimage -destination=/tmp/emcli -platform=”Linux x86-64″ -version=13.4.0.0.0
=== Partition Detail ===
Space free : 4 GB
Space required : 1 GB
Check the logs at /u01/oracle/gc_inst/em/EMGC_OMS1/sysman/emcli/setup/.emcli/get_agentimage_2020-05-27_13-55-05-PM.log
Downloading /tmp/emcli/13.4.0.0.0_AgentCore_226.zip
File saved as /tmp/emcli/13.4.0.0.0_AgentCore_226.zip
Downloading /tmp/emcli/13.4.0.0.0_Plugins_226.zip
File saved as /tmp/emcli/13.4.0.0.0_Plugins_226.zip
Downloading /tmp/emcli/unzip
File saved as /tmp/emcli/unzip
Executing command: /tmp/emcli/unzip /tmp/emcli/13.4.0.0.0_Plugins_226.zip -d /tmp/emcli
Exit status is:0
Agent Image Download completed successfully.

Now copy the newly generated agent to the target server and use it for installing the agent:
# scp /tmp/emcli/13.4.0.0.0_AgentCore_226.zip oracle@SRV1:/u01/oracle/agent13c

On the Target server install the copied agent software:

# mkdir /backup/tmp
# cd /u01/oracle/agent13c
# unzip 13.4.0.0.0_AgentCore_226.zip
# /u01/oracle/agent13c/agentDeploy.sh AGENT_BASE_DIR=/u01/oracle/agent13c
-force
-ignorePrereqs
-invPtrLoc /etc/oraInst.loc 
AGENT_PORT=3872
EM_UPLOAD_PORT=4903
OMS_HOST=OMSSRV
ORACLE_HOSTNAME=SRV1
AGENT_INSTANCE_HOME=/u01/oracle/agent13c/agent_inst
AGENT_REGISTRATION_PASSWORD=xxxx
SCRATCHPATH=/backup/tmp

At the end of installation run this script: [By root]
# /u01/oracle/agent13c/agent_13.4.0.0.0/root.sh           

Make sure the agent is running:
# /u01/oracle/agent13c/agent_13.4.0.0.0/bin/emctl status agent

Conclusion:
As a rule of thumb, always generate a fresh agent on the OMS server to use for installing the EM agent on the target server.