You have the free version of the app - it is a limited version that only allows you to store the latest set of results and do simple calculations.
The paid version has all the features. TFP works with a developer on the App and the amount of time and work that goes into designing it, keeping it updated, and deploying it, as well as running the database on the back end that stores and maintains all of the users data is far more expensive than what the annual subscription pulls in. Supporting the App with an annual subscription ensures that it stays up to date and gives users the features that they request.
Hopefully your time here has shown you how valuable TFP can be as a DIY resource to pool owners worldwide. If you do consider TFP to be a good resource that is free to all who come here, please consider supporting the site as well so that others can benefit from all that TFP has to offer.