The Configure MongoDB Agent to Use TLS page has some inaccurate wording:

For step #1 (Install your certificate file to your Ops Manager configuration directory):

• There is no mongodb-mms-agent system user in Windows, so the example shouldn't display under the Windows tab.
• For the RHEL/CentOS/SLES/AMZ tab, the system user should be mongod.
• For Ubuntu/Debian, the system user should be mongodb.
• For the Linux tab, assuming this means the agent was installed using the TAR archive, the system user should be whatever username the agent runs as. (The TAR archive doesn't create a mongodb-mms-agent system user either.)

For step #3 (Restart the MongoDB Agent for the configuration update to take effect), the agent's service name is mongodb-mms-automation-agent not mongodb-mms-mongodb-agent.