The reason for that message is to upload and install an HTML5 Template into the WordPress environment. You can not install an HTML5 template as a WordPress theme. In that case, you can contact us and purchase the WordPress version of that particular template (If available).
Please note that according to the Envato refund policy, we are not allowed to refund the amount for a mistake purchase. For more detail check the link below-
Our advice is to check the product preview image, the title, and the description accurately before purchasing the item. Additionally, please review the category and price of the product to avoid that kind of issue. Hopefully, that information helps and let us know for further assistance.