start mongod by service mongod start, attempt to create a lock file on a read-only directory /mgodb
If you look that /etc/mongod.conf, what are values of processManagement.pidFilePath and storage.dbPath? It sounds like that system tries to create files to some subdirectory, what don't exists or is not owned by user what is used to start mongod process.

You should never start your mongod with root user. And if you have done that, f.ex. when starting mongod to maintenance mode, always remember to chown -R user: &lt;dir&gt; data files back to the right owner.