mongod.service: Failed with result 'exit-code'
Looks familiar... If you check your mongod.service file (/lib/systemd/system/mongod.service), there should be line what tells where mongod.pid file is located (PIDFile=/tmp/mongod.pid). That same file (with path) should be found from your /etc/mongod.conf file.
processManagement:
  pidFilePath: /tmp/mongod.pid

Problem is that, after 5 second when you have said 'sudo systemctl start mongod.service', system will check &quot;is there mongod.pid&quot; and because it cannot find it, it will KILL that running mongod-process.