The problem is that your directory which you created,
/data/db is owned by the root and only writable by your root user, and you are trying to run mongod as yourself. There are many ways to solve your issue, but ultimately, you should give your directory in question the correct permissions. If this is for the production, then I must advise you to check your docs and think again on it very carefully, you may want to take the special care.
But if this is just for the testing purpose and you just need to do this to work and get on with it, then you must try below command, which will make your directory writable by everyone:
> sudo chmod -R go+w /data/db
Or below command, which will make your directory owned by you:
> sudo chown -R $USER /data/db