Actually, the license which you will get after purchasing the WP Travel Pro plugin will only work for a single domain. You cannot use the same license key for more than one site.
If you use the same license key in the new site which you have previously used on another site, it will generate an issue.
For more detail regarding WP Travel Pro plugin for Multiple sites, please refer to our blog.
Further, if you have any queries, please submit them to our Contact page.