First of all, thank you so much buying the pro version of the plugin.
Actually, WP Travel Pro is a premium addon which includes all the addons of WP Travel plugin. As WP Travel Pro is an addon to WP Travel plugin, it requires the WP Travel plugin to be installed in your site.
Please install WP Travel plugin which is absolutely free by going to, Admin Panel > Plugins > Add New > WP Travel
After adding the plugin, activate it and then try activating WP Travel Pro. This should solve the issue.
Hope this helps
Feel free to reach us for any further query/confusion.