As IP.Board is paid closed-source software, the best way to get help would be to open a support ticket with the team at Invision Power. Their support is part of what you've paid for! That said, the first thing to do is to make sure that your Droplet has everything needed to run IP.Board. There is a PHP script that they provide to do just that.
After you've uploaded the script to your server, move it to the Nginx web root. By default, that is located at
/usr/share/nginx/html Once it's there, you can visit in your browser by going to
http://your.ip.address/ips4.php It will automatically detect any missing components.
If everything checks out, upload the IP.Board and move it to the web root as well. The installer should now be available at
Hopefully that points you in the right direction. Check out their full installation tutorial for more information. You'll also want to take a look at some of the tutorials here on Nginx so you can configure it to suite your needs.