I found similar issue documented in below MOS note which you can review from your side,
Database instance target status at 'Metric Collection Error' in Enterprise Manager Cloud Control 184.108.40.206 (Doc ID 1519711.1)
- Check if the DB plugin available on the agent monitoring the target ? - Output of below command will let us know the same.
<AGENT_HOME>/agent_inst/bin/emctl listplugins agent
- Also check if the Agent is certified on this target server? - You can view the agent certification by logging into MyOracleSupport -> "Certification".
Bookmark the clear cut steps for reviewing certifications :
Please refer following MOS document for the complete certification information.
Understanding Enterprise Manager Certification (Certify) in My Oracle Support [ID 406906.1]
Going forward, you can refer the EM certification matrix from the following.
Login into My Oracle Support
Click Certifications Tab
Choose Enterprise Manager Base Platform OMS or Agent