User Synch Problem

I am trying to make this site a subsite and every time I select the synch button I get the following error:

There was a connection problem. Please check the URL and Key of the Master site.

The URL and Key is correct, I've checked it multiple times. I also have the "Don't overwrite any existing users..." option selected.

Not sure what the issue is. I'm running the latest version of Wordpress and User Sync on both sites.