It says I need a license key for the Tube Ace Plugin.

If you have purchased, your license key is in the receipt email. To activate the Plugin, go to Plugins > Tube Ace License from within WordPress Admin. Enter your license key to activate the plugin for your site.

Thumbnails are not generating.

Make sure you have GD installed on your server by testing at WordPress Admin > Tube Ace > Test Server

Submitted pages to Reddit, Twitter or FaceBook show the wrong thumbnail in the preview.

Your pages must have the Open Graph data in the meta tags. Try this free plugin.