Use normal user and use sudo when you need root privileges
Your mongod command output asks you to run the command without fork.Did you try this?
Also check mongod.log
It will give more details on why it is failing to start
Try to spin up your own mongod using a different port,dbpath,logpath like below
mongod --port 29000 --dbpath your_home_dir --logpath your_home_dir --fork
If it is working you can check/troubleshoot why it is failing with default config file(/etc/mongod.conf)