No option to add online accounts under system settings.
Online accounts hasn't been released into stable yet. You won't have it on a clean install. It's still in development.
It should be right there next to the Network icon:
First of all, verify that you have appropriate packages installed. You might have accidentally removed some of them. Here's the list of packages you need:
The most important one is the first one (
libpantheon-online-accounts0) so you could try fixing this error by typing:
sudo apt-get install --reinstall libpantheon-online-accounts0
If that doesn't work, you should try running
switchboard from the terminal and see if you'll get any errors when executing it. If so, please update your question and post the errors you're getting and I'll do my best to see what went wrong.