I have been working on a website for about a month now and I go to add a plugin to the site and the "add plugin" button on my WP dashboard has vanished, Is there something that can be done to resolve this. Highly frustrating.
Are you sure you haven't been hacked? Check the wp-config.php for any encrypted lines of code. If you find out of ordinary code there, changes are it's been injected into other core .php files too.
If not a conflict, this is the only other situation that I have personally run into to cause what you describe.