A brief word on how vim-plug works...
The main part of a plugin config is a path on GitHub. In your case
dracula/vim refers to https://github.com/dracula/vim. What vim-plug needs to do, then, is download/install (using git) the plugin code from that address. But it doesn't happen automatically. It's up to you to run the command
:PlugInstall. (And do so each time you add a new plugin(s).)
So once you've updated your vimrc just run
:PlugInstall from an instance of Vim that sees the changed vimrc. You'll get some feedback on what happens from there.
Beyond installs there are a number of other commands related to maintaining plugins (
:PlugClean, etc.) that you'll need to keep in mind and run as needed. (The automatic part of vim-plug is primarily the management of runtime paths. The location of each configured plugin is added to that behind the scenes.)