If you can't select "Purge" on your server settings, what does that mean?

I cannot view this option on the PvP server I’m on and also I’m not able to view it in single player. I am trying to experience the Purge.

You have to be the “Admin” to select “Purge” server settings. Do you have “Admin rights”? In Single Player, have you set the difficulty settings to “Custom”? The Purge is in the very last setting (look on the left column for categories of options and you’ll see the Purge listed at the bottom).

I’m not an admin on the PvP server but I am in single player. So when I set the difficulty to custom on single player, will I then be able to select the Purge menu option?

In single player, yes, you should be able to select the Purge menu option in custom difficulty.