PitaJ is correct, that's a text file that you need to edit. The description that you wrote tells all of the details... the two lines given are to be appended to the end of that file. Means the same thing on Linux as it does on Windows or macOS: open a text editor and stick those on the bottom of that file. You'll need to restart MongoDB after you make that change for it to pick it up.

@julian Issue fixed after updating node.js from v6 to v8. But Nodebb installed on localhost. How to transfer that to droplet server ip address

Hello all, Because of budget issues, I would like to install nodebb on shared hosting only. I have access to shell. I guess I can install if i have access to shell. Could you please say procedure as I can't find anywhere. Namecheap shared hosting uses cloudlinux OS and gives users to access shell